Use the DateAdd() function in a formula field.

For example: subtract one year from the today's date: DateAdd("yyyy", -1, CurrentDate)
If today's date is 4/1/2015 this formula will return 4/1/2014.

For example: subtract 30 days from 4/1/2015: DateAdd("d", -30, Date(2015, 04, 01))
This formula will return 3/2/2015.