Allow negative time periods to be specified.

This can be used for ordinal tag filters, for instance for getting the
todo items with a date older than 6 months:

topydo ls 'due:<-6m'

test/RelativeDateTest.py

@@ -62,10 +62,6 @@ class RelativeDateTester(TopydoTest.TopydoTest):
         result = relative_date_to_date('0dd')
         self.assertFalse(result)
 
-    def test_garbage2(self):
-        result = relative_date_to_date('-0d')
-        self.assertFalse(result)
-
     def test_one_day_capital(self):
         result = relative_date_to_date('1D')
         self.assertEquals(result, self.tomorrow)
@@ -111,5 +107,13 @@ class RelativeDateTester(TopydoTest.TopydoTest):
         result = relative_date_to_date('1d', self.tomorrow)
         self.assertEquals(result, date.today() + timedelta(2))
 
+    def test_negative_period1(self):
+        result = relative_date_to_date('-1d')
+        self.assertEquals(result, date.today() - timedelta(1))
+
+    def test_negative_period2(self):
+        result = relative_date_to_date('-0d')
+        self.assertTrue(result, self.today)
+
 if __name__ == '__main__':
     unittest.main()

topydo/lib/RelativeDate.py

@@ -80,7 +80,7 @@ def relative_date_to_date(p_date, p_offset=date.today()):
     result = None
     p_date = p_date.lower()
 
-    relative = re.match('(?P<length>[0-9]+)(?P<period>[dwmy])$', p_date, re.I)
+    relative = re.match('(?P<length>-?[0-9]+)(?P<period>[dwmy])$', p_date, re.I)
 
     monday = 'mo(n(day)?)?$'
     tuesday = 'tu(e(sday)?)?$'