Estimated Close Date Workflow

Hi,

I have had a look at this with little success.

My requirement is to check estimated close date on an opportunity and update by 30 days if the opportunity close date has lapsed and the opportunity is still open.

I had the logic down but i cant find out how to change the estimated close date (advance the date by 30)

Any suggestions on tackling this, i couldnt figure out where the what conditions i needed to advance the date.

Thanks