Date calculation with system date minus 45 days

Hello Experts,

  We have a requirement where we need to default date range to bex query filter by calculating from date as system date minus 45 days and to date as system date for example "20140215 - 20140330".  How can we achieve this in Design Studio 1.2 SP01.  I know we can do this using BEx variables, but I am curious by using Java Script if I can calculate dates and pass to BEx Query.  Thanks in advance.

Thanks

Mohan Kolli