Function to Return 1st of Month

I have a form in Access with 2 date fields for completion. I want to query data from the 1st of the month (e.g. 01 April, 2007) to yesterday. For yesterday I use '=Date()-1', which works great but can't figure out what to use for the other date field.

Will I need to write a function to return this date? Will the function use 'Date' to retrieve the information or will it use the date field box name?

Hope you can help with this, it is driving me crazy!

This will return the first day of the current month of the current year.