Use months in timestamps

Shows timestamps like '2 months ago' instead of '67 days ago', much more
useful. Displays month if greater than 30 days, otherwise shows
days/hours/minutes as it would before. Added coresponding message.
Replaced 'w' with 'mo' in ::DateDiff, as the 'w' was not actually used.

Change-Id: Icab21009c093064b0670c2a5448ba11d740e12f6