matplotlib · jklymak · Jan 19, 2018 · Dec 22, 2017 · Dec 22, 2017 · Dec 22, 2017
diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
@@ -96,11 +96,12 @@
       <../gallery/ticks_and_spines/date_demo_rrule.html>`_.
 
     * :class:`AutoDateLocator`: On autoscale, this class picks the best
-      :class:`RRuleLocator` to set the view limits and the tick
+      :class:`DateLocator` (e.g., :class:`RRuleLocator`)
+      to set the view limits and the tick
       locations.  If called with ``interval_multiples=True`` it will
       make ticks line up with sensible multiples of the tick intervals.  E.g.
       if the interval is 4 hours, it will pick hours 0, 4, 8, etc as ticks.
-      This behaviour is not garaunteed by default.
+      This behaviour is not guaranteed by default.
 
 Date formatters
 ---------------
@@ -128,6 +129,7 @@
 import functools
 
 import warnings
+import logging
 
 
 from dateutil.rrule import (rrule, MO, TU, WE, TH, FR, SA, SU, YEARLY,
@@ -157,6 +159,9 @@
            'seconds', 'minutes', 'hours', 'weeks')
 
 
+_log = logging.getLogger(__name__)
+
+
 # Make a simple UTC instance so we don't always have to import
 # pytz.  From the python datetime library docs:
 
@@ -286,14 +291,21 @@ def _from_ordinalf(x, tz=None):
 
     remainder = float(x) - ix
 
-    # Round down to the nearest microsecond.
-    dt += datetime.timedelta(microseconds=int(remainder * MUSECONDS_PER_DAY))
+    # Since the input date `x` float is unable to preserve microsecond
+    # precision of time representation in non-antique years, the
+    # resulting datetime is rounded to the nearest multiple of
+    # `musec_prec`. A value of 20 is appropriate for current dates.
+    musec_prec = 20
+    remainder_musec = int(round(remainder * MUSECONDS_PER_DAY /
+                                float(musec_prec)) * musec_prec)
 
-    # Compensate for rounding errors
-    if dt.microsecond < 10:
-        dt = dt.replace(microsecond=0)
-    elif dt.microsecond > 999990:
-        dt += datetime.timedelta(microseconds=1e6 - dt.microsecond)
+    # For people trying to plot with full microsecond precision, enable
+    # an early-year workaround
+    if x < 30 * 365:
+        remainder_musec = int(round(remainder * MUSECONDS_PER_DAY))
+
+    # add hours, minutes, seconds, microseconds
+    dt += datetime.timedelta(microseconds=remainder_musec)
 
     return dt.astimezone(tz)
 
@@ -1296,6 +1308,11 @@ def get_locator(self, dmin, dmax):
             locator = RRuleLocator(rrule, self.tz)
         else:
             locator = MicrosecondLocator(interval, tz=self.tz)
+            if dmin.year > 20 and interval < 1000:
+                _log.warn('Plotting microsecond time intervals is not'
+                          ' well supported. Please see the'
+                          ' MicrosecondLocator documentation'
+                          ' for details.')
 
         locator.set_axis(self.axis)
 
@@ -1511,7 +1528,21 @@ def __init__(self, bysecond=None, interval=1, tz=None):
 
 class MicrosecondLocator(DateLocator):
     """
-    Make ticks on occurances of each microsecond.
+    Make ticks on regular intervals of one or more microsecond(s).
+
+    .. note::
+
+        Due to the floating point representation of time in days since
+        0001-01-01 UTC (plus 1), plotting data with microsecond time
+        resolution does not work well with current dates.
+
+        If you want microsecond resolution time plots, it is strongly
+        recommended to use floating point seconds, not datetime-like
+        time representation.
+
+        If you really must use datetime.datetime() or similar and still
+        need microsecond precision, your only chance is to use very
+        early years; using year 0001 is recommended.
 
     """
     def __init__(self, interval=1, tz=None):

diff --git a/...matplotlib/tests/baseline_images/test_dates/DateFormatter_fractionalSeconds.png b/...matplotlib/tests/baseline_images/test_dates/DateFormatter_fractionalSeconds.png
diff --git a/lib/matplotlib/tests/test_dates.py b/lib/matplotlib/tests/test_dates.py
@@ -355,7 +355,6 @@ def _create_auto_date_locator(date1, date2):
                 ['1990-01-01 00:00:00+00:00', '1990-01-01 00:05:00+00:00',
                  '1990-01-01 00:10:00+00:00', '1990-01-01 00:15:00+00:00',
                  '1990-01-01 00:20:00+00:00']
-
                 ],
                [datetime.timedelta(seconds=40),
                 ['1990-01-01 00:00:00+00:00', '1990-01-01 00:00:05+00:00',
@@ -365,11 +364,11 @@ def _create_auto_date_locator(date1, date2):
                  '1990-01-01 00:00:40+00:00']
                 ],
                [datetime.timedelta(microseconds=1500),
-                ['1989-12-31 23:59:59.999507+00:00',
+                ['1989-12-31 23:59:59.999500+00:00',
                  '1990-01-01 00:00:00+00:00',
-                 '1990-01-01 00:00:00.000502+00:00',
-                 '1990-01-01 00:00:00.001005+00:00',
-                 '1990-01-01 00:00:00.001508+00:00']
+                 '1990-01-01 00:00:00.000500+00:00',
+                 '1990-01-01 00:00:00.001000+00:00',
+                 '1990-01-01 00:00:00.001500+00:00']
                 ],
                )
 
@@ -438,11 +437,11 @@ def _create_auto_date_locator(date1, date2):
                  '1997-01-01 00:00:40+00:00']
                 ],
                [datetime.timedelta(microseconds=1500),
-                ['1996-12-31 23:59:59.999507+00:00',
+                ['1996-12-31 23:59:59.999500+00:00',
                  '1997-01-01 00:00:00+00:00',
-                 '1997-01-01 00:00:00.000502+00:00',
-                 '1997-01-01 00:00:00.001005+00:00',
-                 '1997-01-01 00:00:00.001508+00:00']
+                 '1997-01-01 00:00:00.000500+00:00',
+                 '1997-01-01 00:00:00.001000+00:00',
+                 '1997-01-01 00:00:00.001500+00:00']
                 ],
                )