I suspect... he's trying to get a "date" in writing that precedes your actual separation date.
There's a "magic 10" number in many family courts.
At the "magic 10"... spousal support is on the table.
I'm a little jaded after 11 months of reeeeally ugly divorce, so I could be wrong, but that's my first thought. That he doesn't actually want to move out, yet (or pay you anything, arrange for childcare, do his own laundry, etc.), but by the time he thinks he'll be ready to, it will be either too close to, or AFTER your 10 year anniversary.
DO NOT DO NOT DO NOT agree with him.
Ideally... shoot back a "Funny. Haha." kind of response. AKA get it in writing that you aren't agreeing with him.
DO speak to an attorney. PRONTO. As in, research (call up everyone you know who might know a good divorce/family law attorney) this weekend, and go in on monday.
YES. EITHER OF YOU CAN TAKE THE KIDS AND LEAVE... until an emergency custody order is in place. It's 100% legal for EITHER of you to just walk off with the kids.