finding if a period is multiples of a given interval

Hi,
I have two variables in pl/pgsql function.
p_fromdate and p_todate

I have another variable which represents intervals like day, month, quarter
etc.
p_interval as smallint, to hold values like 1,2,3, which are substituted
for intervals as '1 day', '1 month - 1 day', '3 months - 1 day'
respectively.
Now, I have to find if the period of given two dates (p_todate -
p_fromdate) is multiples of the given interval or not?

e.g. p_fromdate = '01/04/2010';
p_todate = '31/03/2013';

p_interval=3 (which is a quarter).

I need to find out if the period of ('31/03/2013' - '01/04/2010') clearly
multiple of a quarter and modulus = 0.
Important point is user can enter any dates and choose any interval to
check. 'Day' interval fits to any dates. For 'month' and others, number of
days, minutes, seconds are varying. So we can not use the fixed values for
them neither we can use '1 month - 1 day' or any interval in division. Also
we can not cast them to integers.

How to get it done?