Make the tag command convert relative dates

When the due date tag or the start date tag is set, convert the value to
an absolute date if a relative date was given.

test/test_tag_command.py

@@ -14,6 +14,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+from freezegun import freeze_time
 import unittest
 
 from test.command_testcase import CommandTest
@@ -138,6 +139,29 @@ class TagCommandTest(CommandTest):
                          "|  4| Fnord due:2014-10-20 due:2014-10-20\n")
         self.assertEqual(self.errors, "")
 
+    @freeze_time('2015, 11, 19')
+    def test_set_tag11(self):
+        command = TagCommand(["3", "due", "today"], self.todolist, self.out,
+                             self.error)
+        command.execute()
+
+        self.assertTrue(self.todolist.is_dirty())
+        self.assertEqual(self.output, "|  3| Baz due:2015-11-19\n")
+        self.assertEqual(self.errors, "")
+
+    def test_set_tag12(self):
+        """
+        Do not convert relative dates for tags that were not configured as
+        start/due date.
+        """
+        command = TagCommand(["3", "foo", "today"], self.todolist, self.out,
+                             self.error)
+        command.execute()
+
+        self.assertTrue(self.todolist.is_dirty())
+        self.assertEqual(self.output, "|  3| Baz due:2014-10-20 foo:today\n")
+        self.assertEqual(self.errors, "")
+
     def test_rm_tag01(self):
         command = TagCommand(["1", "due"], self.todolist, self.out, self.error)
         command.execute()

topydo/commands/TagCommand.py

@@ -15,7 +15,9 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 from topydo.lib.Command import Command, InvalidCommandArgument
+from topydo.lib.Config import config
 from topydo.lib.prettyprinters.Numbers import PrettyPrinterNumbers
+from topydo.lib.RelativeDate import relative_date_to_date
 from topydo.lib.TodoListBase import InvalidTodoException
 
 
@@ -90,7 +92,16 @@ class TagCommand(Command):
 
         return answer
 
+    def _convert_relative_dates(self):
+        if self.tag == config().tag_start() or self.tag == config().tag_due():
+            real_date = relative_date_to_date(self.value)
+
+            if real_date:
+                self.value = real_date.isoformat()
+
     def _set_helper(self, p_old_value=""):
+        self._convert_relative_dates()
+
         old_src = self.todo.source()
         self.todo.set_tag(self.tag, self.value, self.force_add, p_old_value)