Add compare_date/2,3 for comparing only the date or time portion of a provided date/time

[choptastic]

Perhaps the introduction of a function compare_date/2,3 and/or compare_time/2/3.

To demonstrate the thought:

A = "2013-09-01 10am CST",
B = "2013-09-01 12pm CST",
C = "2013-09-02 12am CST",
D = "2013-09-02 12am EST",

qdate:compare_date(A,B).
%% returns 0

qdate:compare_date(A,C).
%% returns -1

qdate:compare_date(C,D).
%% returns ??. 

That last case illustrates the major issue with date comparison.

If C is converted to EST, then it's 2013-09-02 1am, where it would be the same day, whereas if D is converted to CST, then it's 2013-09-01 11pm, in which case it would be different days.

Perhaps the solution is to accept a "Reference Timezone" which both dates would be converted to, or if none is provided to use the timezone stored with set_timezone (which would fall back to GMT).

I'm not entirely sure this is a feasible proposition, and might add too much "magic".

I'd say we should just extract the date portion without even considering the timezone, but what happens when comparing against a unix timestamp or now tuple? Then we would still need to do something with implicit timezones anyway.