Segment based on dates

Hi,

I want to send a reminder by email to contacts who have not completed a task during the current month.

The only element I have is the date at which the contact last completed the task: either in the past month (reminder needs to be sent) or in the current month (no reminder to be sent).

The reminder will be sent every month by an AP. I therefore have to build a segment based on a relative date. How can I build a query based on the first day of the current month? The only items in the relative date dropdown are: current day, x days ago, x weeks ago, x months ago, x days ahead, x weeks ahead, x months ahead.

Can anyone think of a workaround? Any help will be much appreciated!

François