Is it possible to call a VBA function from a cell on a worksheet?

For example:
- Cell A1 contains a date.
- I want Cell A2 to evaluate the date in A1 and then display the date
corresponding to the end of that financial quarter.
- So if A1 = February 23, 2005, I want A2 to return March 31, 2005.

I can write a Function in VBA that would return this value, I just don't
know how I would call it from within a cell on an Excel worksheet and pass
the value of A1.

Any help is appreciated.

Cheers,
Don