how to get the 30 days before date from Todays Date

T-SQL

declare @thirtydaysago datetime
declare @now datetime
set @now = getdate()
set @thirtydaysago = dateadd(day,-30,@now)

select @now, @thirtydaysago

or more simply

select dateadd(day, -30, getdate())

(DATEADD on BOL/MSDN)

MYSQL

SELECT DATE_ADD(NOW(), INTERVAL -30 DAY)

( more DATE_ADD examples on ElectricToolbox.com)