Fix datetime rounding errors.

lib/iris/coords.py

@@ -558,17 +558,13 @@ def _repr_other_metadata(self):
         return result
 
     def _str_dates(self, dates_as_numbers):
-        date_obj_array = self.units.num2date(dates_as_numbers)
+        date_obj_array = np.array(
+            [iris.util._num2date_to_nearest_second(num, self.units)
+             for num in dates_as_numbers])
         kwargs = {'separator': ', ', 'prefix': '      '}
-        try:
-            # With NumPy 1.7 we need to ask for 'str' formatting.
-            result = np.core.arrayprint.array2string(
-                date_obj_array, formatter={'numpystr': str}, **kwargs)
-        except TypeError:
-            # But in 1.6 we don't need to ask, and the option doesn't
-            # even exist!
-            result = np.core.arrayprint.array2string(date_obj_array, **kwargs)
-        return result
+        return np.core.arrayprint.array2string(date_obj_array,
+                                               formatter={'numpystr': str},
+                                               **kwargs)
 
     def __str__(self):
         if self.units.is_time_reference():

lib/iris/fileformats/grib/_save_rules.py

@@ -40,7 +40,7 @@
 from iris.fileformats.grib import grib_phenom_translation as gptx
 from iris.fileformats.grib._load_convert import (_STATISTIC_TYPE_NAMES,
                                                  _TIME_RANGE_UNITS)
-from iris.util import is_regular, regular_step
+from iris.util import is_regular, regular_step, _num2date_to_nearest_second
 
 
 # Invert code tables from :mod:`iris.fileformats.grib._load_convert`.
@@ -604,7 +604,7 @@ def _missing_forecast_period(cube):
         t = t_coord.bounds[0, 0] if t_coord.has_bounds() else t_coord.points[0]
         frt = frt_coord.points[0]
         # Calculate GRIB parameters.
-        rt = frt_coord.units.num2date(frt)
+        rt = _num2date_to_nearest_second(frt, frt_coord.units)
         rt_meaning = 1  # Forecast reference time.
         fp = t - frt
         integer_fp = int(fp)
@@ -619,7 +619,7 @@ def _missing_forecast_period(cube):
         # reference time significance of "Observation time" and set the
         # forecast period to 0h.
         t = t_coord.bounds[0, 0] if t_coord.has_bounds() else t_coord.points[0]
-        rt = t_coord.units.num2date(t)
+        rt = _num2date_to_nearest_second(t, t_coord.units)
         rt_meaning = 3  # Observation time
         fp = 0
         fp_meaning = 1  # Hours

lib/iris/pandas.py

@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2013 - 2015, Met Office
+# (C) British Crown Copyright 2013 - 2016, Met Office
 #
 # This file is part of Iris.
 #
@@ -35,6 +35,7 @@
 import iris
 from iris.coords import AuxCoord, DimCoord
 from iris.cube import Cube
+from iris.util import _num2date_to_nearest_second
 
 
 def _add_iris_coord(cube, name, points, dim, calendar=None):
@@ -119,7 +120,8 @@ def _as_pandas_coord(coord):
     """Convert an Iris Coord into a Pandas index or columns array."""
     index = coord.points
     if coord.units.is_time_reference():
-        index = coord.units.num2date(index)
+        index = np.array([_num2date_to_nearest_second(point, coord.units)
+                          for point in index])
     return index
 
 

lib/iris/plot.py

@@ -47,6 +47,7 @@
 from iris.exceptions import IrisError
 # Importing iris.palette to register the brewer palettes.
 import iris.palette
+from iris.util import _num2date_to_nearest_second
 
 
 # Cynthia Brewer citation text.
@@ -407,7 +408,7 @@ def _fixup_dates(coord, values):
     if coord.units.calendar is not None and values.ndim == 1:
         # Convert coordinate values into tuples of
         # (year, month, day, hour, min, sec)
-        dates = [coord.units.num2date(val).timetuple()[0:6]
+        dates = [_num2date_to_nearest_second(val, coord.units).timetuple()[0:6]
                  for val in values]
         if coord.units.calendar == 'gregorian':
             r = [datetime.datetime(*date) for date in dates]

lib/iris/tests/unit/util/test__num2date_to_nearest_second.py

@@ -0,0 +1,214 @@
+# (C) British Crown Copyright 2016, Met Office
+#
+# This file is part of Iris.
+#
+# Iris is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Lesser General Public License as published by the
+# Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Iris is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with Iris.  If not, see <http://www.gnu.org/licenses/>.
+"""Test function :func:`iris.util._num2date_to_nearest_second`."""
+
+from __future__ import (absolute_import, division, print_function)
+from six.moves import (filter, input, map, range, zip)  # noqa
+
+# import iris tests first so that some things can be initialised before
+# importing anything else
+import iris.tests as tests
+
+import datetime
+
+from cf_units import Unit
+import numpy as np
+import netcdftime
+
+from iris.util import _num2date_to_nearest_second
+
+
+class Test(tests.IrisTest):
+    def setup_units(self, calendar):
+        self.useconds = Unit('seconds since epoch', calendar)
+        self.uminutes = Unit('minutes since epoch', calendar)
+        self.uhours = Unit('hours since epoch', calendar)
+        self.udays = Unit('days since epoch', calendar)
+
+    def check_dates(self, nums, units, expected):
+        for num, unit, exp in zip(nums, units, expected):
+            res = _num2date_to_nearest_second(num, unit)
+            self.assertEqual(exp, res)
+
+    # Gregorian Calendar tests
+
+    def test_simple_gregorian(self):
+        self.setup_units('gregorian')
+        nums = [20., 40.,
+                75., 150.,
+                8., 16.,
+                300., 600.]
+        units = [self.useconds, self.useconds,
+                 self.uminutes, self.uminutes,
+                 self.uhours, self.uhours,
+                 self.udays, self.udays]
+        expected = [datetime.datetime(1970, 1, 1, 0, 0, 20),
+                    datetime.datetime(1970, 1, 1, 0, 0, 40),
+                    datetime.datetime(1970, 1, 1, 1, 15),
+                    datetime.datetime(1970, 1, 1, 2, 30),
+                    datetime.datetime(1970, 1, 1, 8),
+                    datetime.datetime(1970, 1, 1, 16),
+                    datetime.datetime(1970, 10, 28),
+                    datetime.datetime(1971, 8, 24)]
+
+        self.check_dates(nums, units, expected)
+
+    def test_fractional_gregorian(self):
+        self.setup_units('gregorian')
+        nums = [5./60., 10./60.,
+                15./60., 30./60.,
+                8./24., 16./24.]
+        units = [self.uminutes, self.uminutes,
+                 self.uhours, self.uhours,
+                 self.udays, self.udays]
+        expected = [datetime.datetime(1970, 1, 1, 0, 0, 5),
+                    datetime.datetime(1970, 1, 1, 0, 0, 10),
+                    datetime.datetime(1970, 1, 1, 0, 15),
+                    datetime.datetime(1970, 1, 1, 0, 30),
+                    datetime.datetime(1970, 1, 1, 8),
+                    datetime.datetime(1970, 1, 1, 16)]
+
+        self.check_dates(nums, units, expected)
+
+    def test_fractional_second_gregorian(self):
+        self.setup_units('gregorian')
+        nums = [0.25, 0.5, 0.75,
+                1.5, 2.5, 3.5, 4.5]
+        units = [self.useconds]*7
+        expected = [datetime.datetime(1970, 1, 1, 0, 0, 0),
+                    datetime.datetime(1970, 1, 1, 0, 0, 1),
+                    datetime.datetime(1970, 1, 1, 0, 0, 1),
+                    datetime.datetime(1970, 1, 1, 0, 0, 2),
+                    datetime.datetime(1970, 1, 1, 0, 0, 3),
+                    datetime.datetime(1970, 1, 1, 0, 0, 4),
+                    datetime.datetime(1970, 1, 1, 0, 0, 5)]
+
+        self.check_dates(nums, units, expected)
+
+    # 360 day Calendar tests
+
+    def test_simple_360_day(self):
+        self.setup_units('360_day')
+        nums = [20., 40.,
+                75., 150.,
+                8., 16.,
+                300., 600.]
+        units = [self.useconds, self.useconds,
+                 self.uminutes, self.uminutes,
+                 self.uhours, self.uhours,
+                 self.udays, self.udays]
+        expected = [netcdftime.datetime(1970, 1, 1, 0, 0, 20),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 40),
+                    netcdftime.datetime(1970, 1, 1, 1, 15),
+                    netcdftime.datetime(1970, 1, 1, 2, 30),
+                    netcdftime.datetime(1970, 1, 1, 8),
+                    netcdftime.datetime(1970, 1, 1, 16),
+                    netcdftime.datetime(1970, 11, 1),
+                    netcdftime.datetime(1971, 9, 1)]
+
+        self.check_dates(nums, units, expected)
+
+    def test_fractional_360_day(self):
+        self.setup_units('360_day')
+        nums = [5./60., 10./60.,
+                15./60., 30./60.,
+                8./24., 16./24.]
+        units = [self.uminutes, self.uminutes,
+                 self.uhours, self.uhours,
+                 self.udays, self.udays]
+        expected = [netcdftime.datetime(1970, 1, 1, 0, 0, 5),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 10),
+                    netcdftime.datetime(1970, 1, 1, 0, 15),
+                    netcdftime.datetime(1970, 1, 1, 0, 30),
+                    netcdftime.datetime(1970, 1, 1, 8),
+                    netcdftime.datetime(1970, 1, 1, 16)]
+
+        self.check_dates(nums, units, expected)
+
+    def test_fractional_second_360_day(self):
+        self.setup_units('360_day')
+        nums = [0.25, 0.5, 0.75,
+                1.5, 2.5, 3.5, 4.5]
+        units = [self.useconds]*7
+        expected = [netcdftime.datetime(1970, 1, 1, 0, 0, 0),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 1),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 1),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 2),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 3),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 4),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 5)]
+
+        self.check_dates(nums, units, expected)
+
+    # 365 day Calendar tests
+
+    def test_simple_365_day(self):
+        self.setup_units('365_day')
+        nums = [20., 40.,
+                75., 150.,
+                8., 16.,
+                300., 600.]
+        units = [self.useconds, self.useconds,
+                 self.uminutes, self.uminutes,
+                 self.uhours, self.uhours,
+                 self.udays, self.udays]
+        expected = [netcdftime.datetime(1970, 1, 1, 0, 0, 20),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 40),
+                    netcdftime.datetime(1970, 1, 1, 1, 15),
+                    netcdftime.datetime(1970, 1, 1, 2, 30),
+                    netcdftime.datetime(1970, 1, 1, 8),
+                    netcdftime.datetime(1970, 1, 1, 16),
+                    netcdftime.datetime(1970, 10, 28),
+                    netcdftime.datetime(1971, 8, 24)]
+
+        self.check_dates(nums, units, expected)
+
+    def test_fractional_365_day(self):
+        self.setup_units('365_day')
+        nums = [5./60., 10./60.,
+                15./60., 30./60.,
+                8./24., 16./24.]
+        units = [self.uminutes, self.uminutes,
+                 self.uhours, self.uhours,
+                 self.udays, self.udays]
+
+        expected = [netcdftime.datetime(1970, 1, 1, 0, 0, 5),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 10),
+                    netcdftime.datetime(1970, 1, 1, 0, 15),
+                    netcdftime.datetime(1970, 1, 1, 0, 30),
+                    netcdftime.datetime(1970, 1, 1, 8),
+                    netcdftime.datetime(1970, 1, 1, 16)]
+
+        self.check_dates(nums, units, expected)
+
+    def test_fractional_second_365_day(self):
+        self.setup_units('365_day')
+        nums = [0.25, 0.5, 0.75,
+                1.5, 2.5, 3.5, 4.5]
+        units = [self.useconds]*7
+        expected = [netcdftime.datetime(1970, 1, 1, 0, 0, 0),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 1),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 1),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 2),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 3),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 4),
+                    netcdftime.datetime(1970, 1, 1, 0, 0, 5)]
+
+        self.check_dates(nums, units, expected)
+
+if __name__ == '__main__':
+    tests.main()

lib/iris/util.py

@@ -1589,3 +1589,35 @@ def demote_dim_coord_to_aux_coord(cube, name_or_coord):
     cube.remove_coord(dim_coord)
 
     cube.add_aux_coord(dim_coord, coord_dim)
+
+
+def _num2date_to_nearest_second(time_value, units):
+    # Return datetime encoding of numeric time value with respect to the given
+    # time reference units, with a resolution of 1 second.
+
+    # We account for the edge case where the time is in seconds and has a
+    # half second: units.num2date() may produce a date that would round
+    # down.
+    #
+    # Note that this behaviour is different to the num2date function in older
+    # versions of netcdftime that didn't have microsecond precision. In those
+    # versions, a half-second value would be rounded up or down arbitrarily. It
+    # is probably not possible to replicate that behaviour with the current
+    # version (1.4.1), if one wished to do so for the sake of consistency.
+    has_half_second = units.utime().units == 'seconds' and \
+        time_value % 1. == 0.5
+    date = units.num2date(time_value)
+    try:
+        microsecond = date.microsecond
+    except AttributeError:
+        microsecond = 0
+    if has_half_second or microsecond > 0:
+        if has_half_second or microsecond >= 500000:
+            seconds = cf_units.Unit('second')
+            second_frac = seconds.convert(0.75, units.utime().units)
+            time_value += second_frac
+            date = units.num2date(time_value)
+        date = date.__class__(date.year, date.month, date.day,
+                              date.hour, date.minute, date.second)
+
+    return date