Fixed #2443 -- Added DurationField.

A field for storing periods of time - modeled in Python by timedelta. It
is stored in the native interval data type on PostgreSQL and as a bigint
of microseconds on other backends.

Also includes significant changes to the internals of time related maths
in expressions, including the removal of DateModifierNode.

Thanks to Tim and Josh in particular for reviews.

django/forms/fields.py

@@ -26,7 +26,9 @@ from django.forms.widgets import (
 from django.utils import formats
 from django.utils.encoding import smart_text, force_str, force_text
 from django.utils.ipv6 import clean_ipv6_address
+from django.utils.dateparse import parse_duration
 from django.utils.deprecation import RemovedInDjango19Warning, RemovedInDjango20Warning, RenameMethodsBase
+from django.utils.duration import duration_string
 from django.utils import six
 from django.utils.six.moves.urllib.parse import urlsplit, urlunsplit
 from django.utils.translation import ugettext_lazy as _, ungettext_lazy
@@ -37,7 +39,7 @@ from django.core.validators import EMPTY_VALUES  # NOQA
 
 __all__ = (
     'Field', 'CharField', 'IntegerField',
-    'DateField', 'TimeField', 'DateTimeField',
+    'DateField', 'TimeField', 'DateTimeField', 'DurationField',
     'RegexField', 'EmailField', 'FileField', 'ImageField', 'URLField',
     'BooleanField', 'NullBooleanField', 'ChoiceField', 'MultipleChoiceField',
     'ComboField', 'MultiValueField', 'FloatField', 'DecimalField',
@@ -518,6 +520,25 @@ class DateTimeField(BaseTemporalField):
         return datetime.datetime.strptime(force_str(value), format)
 
 
+class DurationField(Field):
+    default_error_messages = {
+        'invalid': _('Enter a valid duration.'),
+    }
+
+    def prepare_value(self, value):
+        return duration_string(value)
+
+    def to_python(self, value):
+        if value in self.empty_values:
+            return None
+        if isinstance(value, datetime.timedelta):
+            return value
+        value = parse_duration(value)
+        if value is None:
+            raise ValidationError(self.error_messages['invalid'], code='invalid')
+        return value
+
+
 class RegexField(CharField):
     def __init__(self, regex, max_length=None, min_length=None, error_message=None, *args, **kwargs):
         """