Merge #519

519: Add support for datetime math r=hgrecco
See #510 

- add _Quantity.to_timedelta()
- modify addition and subtraction with datetimes to convert _Quantity instances to timedeltas before performing the operation
- add tests

pint/quantity.py

@@ -10,6 +10,7 @@
 from __future__ import division, unicode_literals, print_function, absolute_import
 
 import copy
+import datetime
 import math
 import operator
 import functools
@@ -654,13 +655,18 @@ class _Quantity(SharedRegistryObject):
         return self.__class__(magnitude, units)
 
     def __iadd__(self, other):
-        if not isinstance(self._magnitude, ndarray):
+        if isinstance(other, datetime.datetime):
+            return self.to_timedelta() + other
+        elif not isinstance(self._magnitude, ndarray):
             return self._add_sub(other, operator.add)
         else:
             return self._iadd_sub(other, operator.iadd)
 
     def __add__(self, other):
-        return self._add_sub(other, operator.add)
+        if isinstance(other, datetime.datetime):
+            return self.to_timedelta() + other
+        else:
+            return self._add_sub(other, operator.add)
 
     __radd__ = __add__
 
@@ -674,7 +680,10 @@ class _Quantity(SharedRegistryObject):
         return self._add_sub(other, operator.sub)
 
     def __rsub__(self, other):
-        return -self._add_sub(other, operator.sub)
+        if isinstance(other, datetime.datetime):
+            return other - self.to_timedelta()
+        else:
+            return -self._add_sub(other, operator.sub)
 
     @ireduce_dimensions
     def _imul_div(self, other, magnitude_op, units_op=None):
@@ -1511,6 +1520,9 @@ class _Quantity(SharedRegistryObject):
                 is_ok = False
         return is_ok
 
+    def to_timedelta(self):
+        return datetime.timedelta(microseconds=self.to('microseconds').magnitude)
+
 
 def build_quantity_class(registry, force_ndarray=False):
 

pint/testsuite/test_quantity.py

@@ -3,6 +3,7 @@
 from __future__ import division, unicode_literals, print_function, absolute_import
 
 import copy
+import datetime
 import math
 import operator as op
 import warnings
@@ -1205,3 +1206,31 @@ class TestDimensionReduction(QuantityTestCase):
         ureg = UnitRegistry(auto_reduce_dimensions=False)
         x = (10 * ureg.feet) / (3 * ureg.inches)
         self.assertEqual(x.units, ureg.feet / ureg.inches)
+
+
+class TestTimedelta(QuantityTestCase):
+    def test_add_sub(self):
+        d = datetime.datetime(year=1968, month=1, day=10, hour=3, minute=42, second=24)
+        after = d + 3 * self.ureg.second
+        self.assertEqual(d + datetime.timedelta(seconds=3), after)
+        after = 3 * self.ureg.second + d
+        self.assertEqual(d + datetime.timedelta(seconds=3), after)
+        after = d - 3 * self.ureg.second
+        self.assertEqual(d - datetime.timedelta(seconds=3), after)
+        with self.assertRaises(DimensionalityError):
+            3 * self.ureg.second - d
+
+    def test_iadd_isub(self):
+        d = datetime.datetime(year=1968, month=1, day=10, hour=3, minute=42, second=24)
+        after = copy.copy(d)
+        after += 3 * self.ureg.second
+        self.assertEqual(d + datetime.timedelta(seconds=3), after)
+        after = 3 * self.ureg.second
+        after += d
+        self.assertEqual(d + datetime.timedelta(seconds=3), after)
+        after = copy.copy(d)
+        after -= 3 * self.ureg.second
+        self.assertEqual(d - datetime.timedelta(seconds=3), after)
+        after = 3 * self.ureg.second
+        with self.assertRaises(DimensionalityError):
+            after -= d