diff --git a/accounting/views.py b/accounting/views.py
index fd39d8b..837d80d 100644
--- a/accounting/views.py
+++ b/accounting/views.py
@@ -18,6 +18,7 @@
 """The view controllers of the accounting application.
 
 """
+from datetime import date
 
 from django.http import HttpResponseRedirect, HttpResponse
 
@@ -25,11 +26,12 @@ from django.urls import reverse
 from django.utils import dateformat
 from django.utils.decorators import method_decorator
 from django.utils.timezone import localdate
+from django.utils.translation import get_language
 from django.views import generic
 from django.views.decorators.http import require_GET
 
 from accounting.models import Record
-from mia_core.period import PeriodParser
+from mia_core.period import Period
 from mia import settings
 from mia_core.digest_auth import digest_login_required
 from mia_core.utils import UrlBuilder, Pagination, \
@@ -70,12 +72,12 @@ class BaseReportView(generic.ListView):
     Attributes:
         page_no (int): The specified page number
         page_size (int): The specified page size
-        period_parser (PeriodParser): The period parser
+        period (Period): The template period helper
     """
     page_no = None
     page_size = None
     pagination = None
-    period_parser = None
+    period = None
 
     def get(self, request, *args, **kwargs):
         """Adds object_list to the context.
@@ -125,7 +127,7 @@ class BaseReportView(generic.ListView):
 
     def get_context_data(self, **kwargs):
         data = super(BaseReportView, self).get_context_data(**kwargs)
-        data["period"] = self.period_parser
+        data["period"] = self.period
         data["pagination_links"] = self.pagination.links
         return data
 
@@ -142,7 +144,9 @@ class CashReportView(BaseReportView):
         Returns:
             List[Record]: The accounting records for the cash report
         """
-        self.period_parser = PeriodParser(self.kwargs["period_spec"])
+        self.period = Period(
+            get_language(), date(2000, 1, 1), date(2030, 12, 31),
+            self.kwargs["period_spec"])
         if self.kwargs["subject_code"] == "0":
             records = Record.objects.raw(
                 """SELECT r.*
@@ -174,7 +178,7 @@ ORDER BY
   t.ord,
   CASE WHEN is_credit THEN 1 ELSE 2 END,
   r.ord""",
-                [self.period_parser.start, self.period_parser.end])
+                [self.period.start, self.period.end])
         else:
             records = Record.objects.raw(
                 """SELECT r.*
@@ -200,8 +204,8 @@ ORDER BY
   t.ord,
   CASE WHEN is_credit THEN 1 ELSE 2 END,
   r.ord""",
-                [self.period_parser.start,
-                 self.period_parser.end,
+                [self.period.start,
+                 self.period.end,
                  self.kwargs["subject_code"] + "%",
                  self.kwargs["subject_code"] + "%"])
         self.pagination = Pagination(
diff --git a/mia_core/period.py b/mia_core/period.py
index d34b3d1..8e0d4bd 100644
--- a/mia_core/period.py
+++ b/mia_core/period.py
@@ -22,212 +22,509 @@
 import re
 from datetime import date, timedelta
 
+from django.core.serializers.json import DjangoJSONEncoder
 from django.template import defaultfilters
 from django.utils import dateformat
 from django.utils.timezone import localdate
-from django.utils.translation import gettext, pgettext
+from django.utils.translation import gettext
 
 
-class PeriodParser:
-    """The period parser.
+class Period:
+    """The template helper for the period chooser.
 
     Args:
-        spec (str): The period specification.
+        language (str): The current language.
+        data_start (date): The available first day of the data.
+        data_end (date): The available last day of the data.
+        spec (str): The current period specification
 
     Attributes:
-        spec (str): The currently-using period specification.
-        start (date): The start of the period.
-        end (date): The end of the period.
-        description (str): The text description of the period.
-        error (str): The period specification format error, or
-                     None on success.
+        spec (date): The currently-working period specification.
+        start (date): The start day of the currently-specified period.
+        end (date): The end day of the currently-specified period.
+        description (str): The description of the currently-specified
+                           period.
+        this_month (str): The specification of this month.
+        last_month (str): The specification of last month.
+        since_last_month (str): The specification since last month.
+        has_months_to_choose (bool): Whether there are months to
+                                     choose besides this month and
+                                     last month.
+        chosen_month (bool): The specification of the chosen month,
+                             or None if the current period is not a
+                             month or is out of available data range.
+        this_year (str): The specification of this year.
+        last_year (str): The specification of last year.
+        has_years_to_choose (bool): Whether there are years to
+                                    choose besides this year and
+                                    last year.
+        years_to_choose (list[str]): This specification of the
+                                     available years to choose,
+                                     besides this year and last year.
+        today (str): The specification of today.
+        yesterday (str): The specification of yesterday.
+        chosen_start (str): The specification of the first day of the
+                            specified period, as the default date for
+                            the single-day chooser.
+        has_days_to_choose (bool): Whether there is a day range to
+                                   choose.
+        data_start (str): The specification of the available first day.
+        data_end (str): The specification of the available last day.
+        chosen_start (str): The specification of the first day of the
+                            specified period
+        chosen_end (str): The specification of the last day of the
+                          specified period
+        month_picker_params (str): The month-picker parameters, as a
+                                   JSON text string
     """
-    spec = None
-    start = None
-    end = None
-    description = None
-    error = None
+    _language = None
+    _data_start = None
+    _data_end = None
+    _period = None
 
-    def __init__(self, spec):
-        self.spec = spec
-        # A specific month
-        m = re.match("^([0-9]{4})-([0-9]{2})$", spec)
-        if m is not None:
-            year = int(m.group(1))
-            month = int(m.group(2))
-            try:
-                self.start = date(year, month, 1)
-            except ValueError:
-                self.invalid_period()
-                return
-            self.end = self.get_month_last_day(self.start)
-            self.description = self.get_month_text(year, month)
-            return
-        # From a specific month
-        m = re.match("^([0-9]{4})-([0-9]{2})-$", spec)
-        if m is not None:
-            year = int(m.group(1))
-            month = int(m.group(2))
-            try:
-                self.start = date(year, month, 1)
-            except ValueError:
-                self.invalid_period()
-                return
-            self.end = self.get_month_last_day(localdate())
-            self.description = gettext(
-                "Since %s") % self.get_month_text(year, month)
-            return
-        # A specific year
-        m = re.match("^([0-9]{4})$", spec)
-        if m is not None:
-            year = int(m.group(1))
-            try:
-                self.start = date(year, 1, 1)
-            except ValueError:
-                self.invalid_period()
-                return
-            self.end = date(year, 12, 31)
-            today = localdate()
-            if year == today.year:
-                self.description = gettext("This Year")
-            elif year == today.year - 1:
-                self.description = gettext("Last Year")
-            else:
-                self.description = str(year)
-            return
-        # All time
-        if spec == "-":
-            self.start = date(2000, 1, 1)
-            self.end = self.get_month_last_day(localdate())
-            self.description = gettext("All")
-            return
-        # A specific date
-        m = re.match("^([0-9]{4})-([0-9]{2})-([0-9]{2})$",
-                     spec)
-        if m is not None:
-            try:
-                self.start = date(
-                    int(m.group(1)), int(m.group(2)), int(m.group(3)))
-            except ValueError:
-                self.invalid_period()
-                return
-            self.end = self.start
-            self.description = self.get_date_text(self.start)
-            return
-        # A specific date period
-        m = re.match(("^([0-9]{4})-([0-9]{2})-([0-9]{2})"
-                      "-([0-9]{4})-([0-9]{2})-([0-9]{2})$"),
-                     spec)
-        if m is not None:
-            try:
-                self.start = date(
-                    int(m.group(1)), int(m.group(2)), int(m.group(3)))
-                self.end = date(
-                    int(m.group(4)), int(m.group(5)), int(m.group(6)))
-            except ValueError:
-                self.invalid_period()
-                return
-            today = localdate()
-            # Spans several years
-            if self.start.year != self.end.year:
-                self.description = "%s-%s" % (
-                    defaultfilters.date(self.start, "Y/n/j"),
-                    defaultfilters.date(self.end, "Y/n/j"))
-            # Spans several months
-            elif self.start.month != self.end.month:
-                if self.start.year != today.year:
-                    self.description = "%s-%s" % (
-                        defaultfilters.date(self.start, "Y/n/j"),
-                        defaultfilters.date(self.end, "n/j"))
-                else:
-                    self.description = "%s-%s" % (
-                        defaultfilters.date(self.start, "n/j"),
-                        defaultfilters.date(self.end, "n/j"))
-            # Spans several days
-            elif self.start.day != self.end.day:
-                if self.start.year != today.year:
-                    self.description = "%s-%s" % (
-                        defaultfilters.date(self.start, "Y/n/j"),
-                        defaultfilters.date(self.end, "j"))
-                else:
-                    self.description = "%s-%s" % (
-                        defaultfilters.date(self.start, "n/j"),
-                        defaultfilters.date(self.end, "j"))
-            # At the same day
-            else:
-                self.spec = dateformat.format(self.start, "Y-m-d")
-                self.description = self.get_date_text(self.start)
-            return
-        # Wrong period format
-        self.invalid_period()
+    def __init__(self, language, data_start, data_end, spec):
+        self._language = language
+        self._data_start = data_start
+        self._data_end = data_end
+        self._period = self.Parser(spec)
 
-    def invalid_period(self):
-        """Sets the period when the period specification is invalid.
-        """
-        self.error = gettext("Invalid period.")
-        today = localdate()
-        self.spec = dateformat.format(localdate(), "Y-m")
-        self.start = date(today.year, today.month, 1)
-        self.end = self.get_month_last_day(self.start)
-        self.description = gettext("This Month")
+    @property
+    def spec(self):
+        return self._period.spec
+
+    @property
+    def start(self):
+        return self._period.start
+
+    @property
+    def end(self):
+        return self._period.end
+
+    @property
+    def description(self):
+        return self._period.description
 
     @staticmethod
-    def get_month_last_day(day):
-        """Calculates and returns the last day of a month.
-
-        Args:
-            day (date): A day in the month.
+    def _get_last_month_start():
+        """Returns the first day of the last month.
 
         Returns:
-            date: The last day in the month
-        """
-        next_month = day.month + 1
-        next_year = day.year
-        if next_month > 12:
-            next_month = 1
-            next_year = next_year + 1
-        return date(next_year, next_month, 1) - timedelta(days=1)
-
-    @staticmethod
-    def get_month_text(year, month):
-        """Returns the text description of a month.
-
-        Args:
-            year (int): The year.
-            month (int): The month.
-
-        Returns:
-            str: The description of the month.
+            date: The first day of the last month.
         """
         today = localdate()
-        if year == today.year and month == today.month:
-            return gettext("This Month")
-        prev_month = today.month - 1
-        prev_year = today.year
-        if prev_month < 1:
-            prev_month = 12
-            prev_year = prev_year - 1
-        prev = date(prev_year, prev_month, 1)
-        if year == prev.year and month == prev.month:
-            return gettext("Last Month")
-        return "%d/%d" % (year, month)
+        month = today.month - 1
+        year = today.year
+        if month < 1:
+            month = 12
+            year = year - 1
+        return date(year, month, 1)
 
     @staticmethod
-    def get_date_text(day):
-        """Returns the text description of a day.
-
-        Args:
-            day (date): The date.
+    def _get_next_month_start():
+        """Returns the first day of the next month.
 
         Returns:
-            str: The description of the day.
+            date: The first day of the next month.
         """
         today = localdate()
-        if day == today:
-            return gettext("Today")
-        elif day == today - timedelta(days=1):
-            return gettext("Yesterday")
-        elif day.year != today.year:
-            return defaultfilters.date(day, "Y/n/j")
+        month = today.month + 1
+        year = today.year
+        if month > 12:
+            month = 1
+            year = year + 1
+        return date(year, month, 1)
+
+    @property
+    def this_month(self):
+        if self._data_start is None:
+            return None
+        today = localdate()
+        first_month_start = date(
+            self._data_start.year, self._data_start.month, 1)
+        if today < first_month_start:
+            return None
+        return dateformat.format(today, "Y-m")
+
+    @property
+    def last_month(self):
+        if self._data_start is None:
+            return None
+        last_month_start = self._get_last_month_start()
+        first_month_start = date(
+            self._data_start.year, self._data_start.month, 1)
+        if last_month_start < first_month_start:
+            return None
+        return dateformat.format(last_month_start, "Y-m")
+
+    @property
+    def since_last_month(self):
+        last_month = self.last_month
+        if last_month is None:
+            return None
+        return self.last_month + "-"
+
+    @property
+    def has_months_to_choose(self):
+        if self._data_start is None:
+            return None
+        if self._data_start < self._get_last_month_start():
+            return True
+        if self._data_end >= self._get_next_month_start():
+            return True
+        return False
+
+    @property
+    def chosen_month(self):
+        if self._data_start is None:
+            return None
+        m = re.match("^[0-9]{4}-[0-2]{2}", self._period.spec)
+        if m is None:
+            return None
+        if self._period.end < self._data_start:
+            return None
+        if self._period.start > self._data_end:
+            return None
+        return self._period.spec
+
+    @property
+    def this_year(self):
+        if self._data_start is None:
+            return None
+        this_year = localdate().year
+        if this_year < self._data_start.year:
+            return None
+        return str(this_year)
+
+    @property
+    def last_year(self):
+        if self._data_start is None:
+            return None
+        last_year = localdate().year - 1
+        if last_year < self._data_start.year:
+            return None
+        return str(last_year)
+
+    @property
+    def has_years_to_choose(self):
+        if self._data_start is None:
+            return None
+        this_year = localdate().year
+        if self._data_start.year < this_year - 1:
+            return True
+        if self._data_end.year > this_year:
+            return True
+        return False
+
+    @property
+    def years_to_choose(self):
+        if self._data_start is None:
+            return None
+        this_year = localdate().year
+        before = [str(x) for x in range(
+            self._data_start.year, this_year - 1)]
+        after = [str(x) for x in range(
+            self._data_end.year, this_year, -1)]
+        return after + before[::-1]
+
+    def is_chosen_year(self, year):
+        """Returne whether the specified year is the currently-chosen
+        year.
+
+        Args:
+            year (str): the year.
+
+        Returns:
+            bool: True if the year is the currently-chosen year, or
+            False otherwise
+        """
+        if self._period.spec == str(year):
+            return True
+
+    @property
+    def today(self):
+        if self._data_start is None:
+            return None
+        today = localdate()
+        if today < self._data_start or today > self._data_end:
+            return None
+        return dateformat.format(today, "Y-m-d")
+
+    @property
+    def yesterday(self):
+        if self._data_start is None:
+            return None
+        yesterday = localdate() - timedelta(days=1)
+        if yesterday < self._data_start or yesterday > self._data_end:
+            return None
+        return dateformat.format(yesterday, "Y-m-d")
+
+    @property
+    def chosen_day(self):
+        return dateformat.format(self._period.start, "Y-m-d")
+
+    @property
+    def has_days_to_choose(self):
+        if self._data_start is None:
+            return False
+        if self._data_start == self._data_end:
+            return False
+        return True
+
+    @property
+    def first_day(self):
+        if self._data_start is None:
+            return None
+        return dateformat.format(self._data_start, "Y-m-d")
+
+    @property
+    def last_day(self):
+        if self._data_end is None:
+            return None
+        return dateformat.format(self._data_end, "Y-m-d")
+
+    @property
+    def chosen_start(self):
+        if self._data_start is None:
+            return None
+        day = self._period.start \
+            if self._period.start >= self._data_start \
+            else self._data_start
+        return dateformat.format(day, "Y-m-d")
+
+    @property
+    def chosen_end(self):
+        if self._data_end is None:
+            return None
+        day = self._period.end \
+            if self._period.end <= self._data_end \
+            else self._data_end
+        return dateformat.format(day, "Y-m-d")
+
+    @property
+    def month_picker_params(self):
+        if self._data_start is None:
+            return None
+        if self._language == "zh-Hant":
+            locale = "zh_TW"
+        elif self._language == "zh-Hans":
+            locale = "zh_CN"
         else:
-            return defaultfilters.date(day, "n/j")
+            locale = self._language.replace("-", "_")
+        start = date(self._data_start.year, self._data_start.month, 1)
+        return DjangoJSONEncoder().encode({
+            "locale": locale,
+            "minDate": start,
+            "maxDate": self._data_end,
+            "defaultDate": self.chosen_month,
+        })
+
+    class Parser:
+        """The period parser.
+
+        Args:
+            spec (str): The period specification.
+
+        Attributes:
+            spec (str): The currently-using period specification.
+            start (date): The start of the period.
+            end (date): The end of the period.
+            description (str): The text description of the period.
+            error (str): The period specification format error, or
+                         None on success.
+        """
+        spec = None
+        start = None
+        end = None
+        description = None
+        error = None
+
+        def __init__(self, spec):
+            self.spec = spec
+            # A specific month
+            m = re.match("^([0-9]{4})-([0-9]{2})$", spec)
+            if m is not None:
+                year = int(m.group(1))
+                month = int(m.group(2))
+                try:
+                    self.start = date(year, month, 1)
+                except ValueError:
+                    self.invalid_period()
+                    return
+                self.end = self.get_month_last_day(self.start)
+                self.description = self.get_month_text(year, month)
+                return
+            # From a specific month
+            m = re.match("^([0-9]{4})-([0-9]{2})-$", spec)
+            if m is not None:
+                year = int(m.group(1))
+                month = int(m.group(2))
+                try:
+                    self.start = date(year, month, 1)
+                except ValueError:
+                    self.invalid_period()
+                    return
+                self.end = self.get_month_last_day(localdate())
+                self.description = gettext(
+                    "Since %s") % self.get_month_text(year, month)
+                return
+            # A specific year
+            m = re.match("^([0-9]{4})$", spec)
+            if m is not None:
+                year = int(m.group(1))
+                try:
+                    self.start = date(year, 1, 1)
+                except ValueError:
+                    self.invalid_period()
+                    return
+                self.end = date(year, 12, 31)
+                today = localdate()
+                if year == today.year:
+                    self.description = gettext("This Year")
+                elif year == today.year - 1:
+                    self.description = gettext("Last Year")
+                else:
+                    self.description = str(year)
+                return
+            # All time
+            if spec == "-":
+                self.start = date(2000, 1, 1)
+                self.end = self.get_month_last_day(localdate())
+                self.description = gettext("All")
+                return
+            # A specific date
+            m = re.match("^([0-9]{4})-([0-9]{2})-([0-9]{2})$",
+                         spec)
+            if m is not None:
+                try:
+                    self.start = date(
+                        int(m.group(1)),
+                        int(m.group(2)),
+                        int(m.group(3)))
+                except ValueError:
+                    self.invalid_period()
+                    return
+                self.end = self.start
+                self.description = self.get_date_text(self.start)
+                return
+            # A specific date period
+            m = re.match(("^([0-9]{4})-([0-9]{2})-([0-9]{2})"
+                          "-([0-9]{4})-([0-9]{2})-([0-9]{2})$"),
+                         spec)
+            if m is not None:
+                try:
+                    self.start = date(
+                        int(m.group(1)),
+                        int(m.group(2)),
+                        int(m.group(3)))
+                    self.end = date(
+                        int(m.group(4)),
+                        int(m.group(5)),
+                        int(m.group(6)))
+                except ValueError:
+                    self.invalid_period()
+                    return
+                today = localdate()
+                # Spans several years
+                if self.start.year != self.end.year:
+                    self.description = "%s-%s" % (
+                        defaultfilters.date(self.start, "Y/n/j"),
+                        defaultfilters.date(self.end, "Y/n/j"))
+                # Spans several months
+                elif self.start.month != self.end.month:
+                    if self.start.year != today.year:
+                        self.description = "%s-%s" % (
+                            defaultfilters.date(self.start, "Y/n/j"),
+                            defaultfilters.date(self.end, "n/j"))
+                    else:
+                        self.description = "%s-%s" % (
+                            defaultfilters.date(self.start, "n/j"),
+                            defaultfilters.date(self.end, "n/j"))
+                # Spans several days
+                elif self.start.day != self.end.day:
+                    if self.start.year != today.year:
+                        self.description = "%s-%s" % (
+                            defaultfilters.date(self.start, "Y/n/j"),
+                            defaultfilters.date(self.end, "j"))
+                    else:
+                        self.description = "%s-%s" % (
+                            defaultfilters.date(self.start, "n/j"),
+                            defaultfilters.date(self.end, "j"))
+                # At the same day
+                else:
+                    self.spec = dateformat.format(self.start, "Y-m-d")
+                    self.description = self.get_date_text(self.start)
+                return
+            # Wrong period format
+            self.invalid_period()
+
+        def invalid_period(self):
+            """Sets the period when the period specification is
+            invalid.
+            """
+            self.error = gettext("Invalid period.")
+            today = localdate()
+            self.spec = dateformat.format(localdate(), "Y-m")
+            self.start = date(today.year, today.month, 1)
+            self.end = self.get_month_last_day(self.start)
+            self.description = gettext("This Month")
+
+        @staticmethod
+        def get_month_last_day(day):
+            """Calculates and returns the last day of a month.
+
+            Args:
+                day (date): A day in the month.
+
+            Returns:
+                date: The last day in the month
+            """
+            next_month = day.month + 1
+            next_year = day.year
+            if next_month > 12:
+                next_month = 1
+                next_year = next_year + 1
+            return date(next_year, next_month, 1) - timedelta(days=1)
+
+        @staticmethod
+        def get_month_text(year, month):
+            """Returns the text description of a month.
+
+            Args:
+                year (int): The year.
+                month (int): The month.
+
+            Returns:
+                str: The description of the month.
+            """
+            today = localdate()
+            if year == today.year and month == today.month:
+                return gettext("This Month")
+            prev_month = today.month - 1
+            prev_year = today.year
+            if prev_month < 1:
+                prev_month = 12
+                prev_year = prev_year - 1
+            prev = date(prev_year, prev_month, 1)
+            if year == prev.year and month == prev.month:
+                return gettext("Last Month")
+            return "%d/%d" % (year, month)
+
+        @staticmethod
+        def get_date_text(day):
+            """Returns the text description of a day.
+
+            Args:
+                day (date): The date.
+
+            Returns:
+                str: The description of the day.
+            """
+            today = localdate()
+            if day == today:
+                return gettext("Today")
+            elif day == today - timedelta(days=1):
+                return gettext("Yesterday")
+            elif day.year != today.year:
+                return defaultfilters.date(day, "Y/n/j")
+            else:
+                return defaultfilters.date(day, "n/j")