Add the Julian day ('g') date pattern

babel/dates.py

@@ -1584,6 +1584,8 @@ def __getitem__(self, name: str) -> str:
             return self.format_day_of_year(num)
         elif char == 'F':
             return self.format_day_of_week_in_month()
+        elif char == 'g':
+            return self.format_julian_day(num)
         elif char in ('E', 'e', 'c'):
             return self.format_weekday(char, num)
         elif char in ('a', 'b', 'B'):
@@ -1721,6 +1723,33 @@ def format_day_of_year(self, num: int) -> str:
     def format_day_of_week_in_month(self) -> str:
         return str((self.value.day - 1) // 7 + 1)
 
+    def format_julian_day(self, num: int) -> str:
+        """
+        Return the modified Julian day.
+
+        This is different from the conventional Julian day number:
+        - It demarcates days at local zone midnight, rather than noon GMT.
+        - It depends on the local time zone.
+
+        >>> from datetime import date
+        >>> format = DateTimeFormat(date(2016, 2, 28), 'cs_CZ')
+        >>> format.format_julian_day(1)
+        '2457447'
+
+        :param num: count of format character
+
+        """
+        # Python defines `dt.toordinal()` which gives us the Gregorian day for
+        # a given date. To compute the Julian day for a given date, we just
+        # need to know the offset between the Julian and Gregorian calendars.
+        # This offset can be computed by taking the `JAN_1_1_JULIAN_DAY`
+        # constant defined in ICU and subtracting `1` from it. This is because
+        # `date(1, 1, 1).toordinal() == 1` since January 1, 1 is the start of
+        # the Gregorian calendar.
+        # See: https://github.com/unicode-org/icu/blob/6d18ecab418ac4cf5a3f51ec3b69b77ba713d719/icu4j/main/core/src/main/java/com/ibm/icu/util/Calendar.java#L1136-L1141
+        JULIAN_DAY_OFFSET = 1721426 - 1
+        return self.format(self.value.toordinal() + JULIAN_DAY_OFFSET, num)
+
     def format_period(self, char: str, num: int) -> str:
         """
         Return period from parsed datetime according to format pattern.

docs/dates.rst

@@ -173,7 +173,7 @@ Date Fields
   |          +--------+--------------------------------------------------------+
   |          | ``F``  | Day of week in month.                                  |
   |          +--------+--------------------------------------------------------+
-  |          | ``g``  | ??                                                     |
+  |          | ``g``  | Modified Julian day (numeric).                         |
   +----------+--------+--------------------------------------------------------+
   | Week day | ``E``  | Day of week. Use one through three letters for the     |
   |          |        | short day, or four for the full name, or five for the  |

tests/test_dates.py

@@ -38,6 +38,23 @@ def test_format_date_with_day_of_year_in_pattern_and_datetime_param():
     assert dates.format_date(d, 'w', locale='en_US') == '14'
 
 
+def test_format_julian_day(timezone_getter):
+    assert dates.format_date(date(476, 9, 4), 'g', locale='en') == '1895163'
+    assert dates.format_date(date(1970, 1, 1), 'g', locale='en') == '2440588'
+    # The time part of the datetime is ignored
+    assert dates.format_date(datetime(1970, 1, 1, 12, 30), 'g', locale='en') == '2440588'
+    # Zero padding
+    assert dates.format_date(date(2026, 11, 7), 'ggggggggg', locale='en') == '002461352'
+    # Local time is used for the calculation
+    # 23:30 UTC on March 31 is already April 1 in Zurich so the
+    # Julian day in the Zurich timezone should be 1 more than UTC
+    dt = datetime(2007, 3, 31, 23, 30, tzinfo=UTC)
+    jd_utc = dates.format_datetime(dt, 'g', locale='en')
+    jd_zurich = dates.format_datetime(dt, 'g', tzinfo=timezone_getter('Europe/Zurich'), locale='en')
+    assert jd_utc == '2454191'
+    assert jd_zurich == f'{int(jd_utc)+1}'
+
+
 def test_format_datetime_with_float(timezone_getter):
     UTC = timezone_getter('UTC')
     d = datetime(2012, 4, 1, 15, 30, 29, tzinfo=UTC)