Return a TTL when formatting times

Add an out parameter to Language::sprintfDate that returns the amount of
time that its output is valid for (e.g., an output format of 'Y-m-d' at
11:50 PM would be valid for 600 seconds).

Change-Id: I3f5a80aa4d303f92c97d24ab780af920894d24ef
Merge "Return a TTL when formatting times"