Date manipulation

How does one perform date manipulation within SQL?  For example, SQL
Server has a dateadd() function that takes a date part, scalar, and the
date to manipulate.

I have a query that determines the number of days that are between now
and a particular date that looks something like this:

select datediff (dd, now (), column) as difference_in_days from ...

Anything available short of coding my own function?

Thanks,

Mark