do -d now understands relative dates

To complete a todo item that you finished yesterday, run

    topydo do -d yesterday

Or

    topydo do -d -1d

test/test_do_command.py

@@ -304,6 +304,32 @@ class DoCommandTest(CommandTest):
         self.assertEqual(self.output, "| 12| {today} Strict due:2014-01-02 rec:1d\nCompleted: x {yesterday} Strict due:2014-01-01 rec:1d\n".format(today=self.today, yesterday=self.yesterday))
         self.assertEqual(self.errors, "")
 
+    def test_do_custom_date8(self):
+        """
+        Convert relative completion dates to an absolute date (yesterday).
+        """
+        command = DoCommand(["-d", "yesterday", "3"], self.todolist, self.out,
+                            self.error)
+        command.execute()
+
+        self.assertTrue(self.todolist.is_dirty())
+        self.assertEqual(self.output,
+                         "Completed: x {} Baz p:1\n".format(self.yesterday))
+        self.assertEqual(self.errors, "")
+
+    def test_do_custom_date9(self):
+        """
+        Convert relative completion dates to an absolute date (-1d)
+        """
+        command = DoCommand(["-d", "-1d", "3"], self.todolist, self.out,
+                            self.error)
+        command.execute()
+
+        self.assertTrue(self.todolist.is_dirty())
+        self.assertEqual(self.output,
+                         "Completed: x {} Baz p:1\n".format(self.yesterday))
+        self.assertEqual(self.errors, "")
+
     def test_multi_do1(self):
         command = DoCommand(["1", "3"], self.todolist, self.out, self.error,
                             _yes_prompt)

topydo/commands/DoCommand.py

@@ -20,6 +20,7 @@ from topydo.lib.DCommand import DCommand
 from topydo.lib.PrettyPrinter import PrettyPrinter
 from topydo.lib.prettyprinters.Numbers import PrettyPrinterNumbers
 from topydo.lib.Recurrence import NoRecurrenceException, advance_recurring_todo
+from topydo.lib.RelativeDate import relative_date_to_date
 from topydo.lib.Utils import date_string_to_date
 
 
@@ -48,7 +49,10 @@ class DoCommand(DCommand):
             self.strict_recurrence = True
         elif p_opt == "-d" or p_opt == "--date":
             try:
-                self.completion_date = date_string_to_date(p_value)
+                self.completion_date = relative_date_to_date(p_value)
+
+                if not self.completion_date:
+                    self.completion_date = date_string_to_date(p_value)
             except ValueError:
                 self.completion_date = date.today()