Seperation question

If I left my marital residence 1 month ago with the intention of not returning can the seperation agreement be back dated to the date I left?

The agreement itself is not back dated, but will include your actual date of separation in the text of the agreement.

And is that date the date used for the 1 year and 1 day requirement? Or is the date the seperation was filed the standard?

The date of separation for purposes of filing the divorce is the day you actually began living separate and apart.