> But, one new question: Will gds32.dll not conflict with Interbase? I

This is why I originally suggested put it to UDF directory.

> don't have it installed in my PC, but I'm thinking about my customers,
> where I intend to use this UDF.

I would not use such ancient library. AFAIK it was written for old dialect 1
that supported only TIMESTAMP, not separate DATE and TIME datatypes,
but the keyword used was DATE, which means the declaration
declare external function f_AgeInMonths date, date
is wrong, it should be
declare external function f_AgeInMonths timestamp, timestamp

Also linking to gds32 is unnecessay - Delphi contains tons of date+time
routines, and converting between TDateTime and Firebird's datatypes
is trivial.

Either write you own library, or try to find something newer.
And do not forget that many things can be easily computed directly,
using date arithmetic and/or built-in EXTRACT function.

Ivan