matplotlib · QuLogic · Jul 24, 2020 · Jul 22, 2020
diff --git a/doc/api/next_api_changes/deprecations/17983-JMK.rst b/doc/api/next_api_changes/deprecations/17983-JMK.rst
@@ -1,8 +1,16 @@
-Reverted deprecation of `~.dates.num2epoch` and `~.dates.epoch2num`
+Reverted deprecation of ``num2epoch`` and ``epoch2num``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 These two functions were deprecated in 3.3.0, and did not return
 an accurate Matplotlib datenum relative to the new Matplotlib epoch
 handling (`~.dates.get_epoch` and :rc:`date.epoch`).  This version
-reverts the deprecation and fixes those functions to work with
-`~.dates.get_epoch`. 
+reverts the deprecation.
+
+Functions ``epoch2num`` and ``dates.julian2num`` use ``date.epoch`` rcParam
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Now `~.dates.epoch2num` and (undocumented) ``julian2num`` return floating point
+days since `~.dates.get_epoch` as set by :rc:`date.epoch`, instead of
+floating point days since the old epoch of "0000-12-31T00:00:00".  If
+needed, you can translate from the new to old values as
+``old = new + mdates.date2num(np.datetime64('0000-12-31'))``
diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
@@ -197,7 +197,13 @@ def _get_rc_timezone():
 Time-related constants.
 """
 EPOCH_OFFSET = float(datetime.datetime(1970, 1, 1).toordinal())
-JULIAN_OFFSET = 1721424.5                         # Julian date at 0001-01-01
+# EPOCH_OFFSET is not used by matplotlib
+JULIAN_OFFSET = 1721424.5  # Julian date at 0000-12-31
+# note that the Julian day epoch is achievable w/
+# np.datetime64('-4713-11-24T12:00:00'); datetime64 is proleptic
+# Gregorian and BC has a one-year offset.  So
+# np.datetime64('0000-12-31') - np.datetime64('-4713-11-24T12:00') = 1721424.5
+# Ref: https://en.wikipedia.org/wiki/Julian_day
 MICROSECONDLY = SECONDLY + 1
 HOURS_PER_DAY = 24.
 MIN_PER_HOUR = 60.
@@ -445,14 +451,20 @@ def julian2num(j):
     Parameters
     ----------
     j : float or sequence of floats
-        Julian date(s)
+        Julian dates (days relative to 4713 BC Jan 1, 12:00:00 Julian
+        calendar or 4714 BC Nov 24, 12:00:00, proleptic Gregorian calendar).
 
     Returns
     -------
     float or sequence of floats
-        Matplotlib date(s)
+        Matplotlib dates (days relative to `.get_epoch`).
     """
-    return np.subtract(j, JULIAN_OFFSET)  # Handles both scalar & nonscalar j.
+    ep = np.datetime64(get_epoch(), 'h').astype(float) / 24.
+    ep0 = np.datetime64('0000-12-31T00:00:00', 'h').astype(float) / 24.
+    # Julian offset defined above is relative to 0000-12-31, but we need
+    # relative to our current epoch:
+    dt = JULIAN_OFFSET - ep0 + ep
+    return np.subtract(j, dt)  # Handles both scalar & nonscalar j.
 
 
 def num2julian(n):
@@ -462,14 +474,19 @@ def num2julian(n):
     Parameters
     ----------
     n : float or sequence of floats
-        Matplotlib date(s)
+        Matplotlib dates (days relative to `.get_epoch`).
 
     Returns
     -------
     float or sequence of floats
-        Julian date(s)
-    """
-    return np.add(n, JULIAN_OFFSET)  # Handles both scalar & nonscalar j.
+        Julian dates (days relative to 4713 BC Jan 1, 12:00:00).
+    """
+    ep = np.datetime64(get_epoch(), 'h').astype(float) / 24.
+    ep0 = np.datetime64('0000-12-31T00:00:00', 'h').astype(float) / 24.
+    # Julian offset defined above is relative to 0000-12-31, but we need
+    # relative to our current epoch:
+    dt = JULIAN_OFFSET - ep0 + ep
+    return np.add(n, dt)  # Handles both scalar & nonscalar j.
 
 
 def num2date(x, tz=None):

diff --git a/lib/matplotlib/tests/test_dates.py b/lib/matplotlib/tests/test_dates.py
@@ -1010,3 +1010,17 @@ def test_epoch2num():
     mdates.set_epoch('1970-01-01T00:00:00')
     assert mdates.epoch2num(86400) == 1.0
     assert mdates.num2epoch(2.0) == 86400 * 2
+
+
+def test_julian2num():
+    mdates._reset_epoch_test_example()
+    mdates.set_epoch('0000-12-31')
+    # 2440587.5 is julian date for 1970-01-01T00:00:00
+    # https://en.wikipedia.org/wiki/Julian_day
+    assert mdates.julian2num(2440588.5) == 719164.0
+    assert mdates.num2julian(719165.0) == 2440589.5
+    # set back to the default
+    mdates._reset_epoch_test_example()
+    mdates.set_epoch('1970-01-01T00:00:00')
+    assert mdates.julian2num(2440588.5) == 1.0
+    assert mdates.num2julian(2.0) == 2440589.5