DateDiff() function

Hi Guys

We are migrating to Postgres. In the current system, we use datediff()
function to get the difference between two dates, e.g. datediff (month,
cast('2013-01-01' as timestamp), cast('2013-02-02' as timestamp) returns 1.

I understand that Postgres has Interval data type so I can achieve the same
with Extract(month from Age(date1, date2)). However, I try to make it so
that the existing SQL can run on both databases without changes. One
possible way is to add a datediff function to Postgres, but the problem is
that month/day/year etc is a keyword not a string like 'month'. I noticed
that Postgres seems to convert Extract(month from current_timestamp) to
date_part('month', current_timestamp), you can also do Extract('month' from
current_timestamp). So it seems internally, Postgres can do the mapping
from month to 'month'. I was wondering if there is a way for me to do the
same for the datediff() function? Any other ideas?

Thanks
Huan