Calculate previous Date from days or weeks range

[eclectus]

I have a form that has two fields - one is a number value and the other a list with 'days' 'weeks' 'months'.

The user enters a number and I have to calculate a previous date based on that input.

eg: the user enter 10 days, so I have to calculate the date of 10 days before the current date. The same if they pick 3 weeks, I would need the Date for three weeks before today. And the same with months.

[ZevToledano]

So build a custom function with two 'ifs' (depending on days/weeks/months), then use the AddDays/AddWeeks/AddMonths functions with the date+numeric fields as the parameters.

[eclectus]

well, I need to AddDays(-10) in my example:
1) does the function work to calculate backwards?
2) how do I use the value of the numeric field and give it a negative sign?

[ZevToledano]

1. Yes
2. 0 - field