DCAA: Do I consider a probation employee after I sign a FJO or after I start my first day at work?

Got a verbal offer from HR that I should expect my FJO and will start sometime in March. I completely understand that DOGE is firing probation employees across the government and coming after DOD next. I want to plan strategically to keep the job until DOGE dissolves in 7/4. In addition, I don't think the laying off of probation employees will be a yearly thing.

Ultimately, I want to become a probation employee with DCAA after DOGE figures out how to and what to cut in the DOD. Therefore, do I consider a probation employee after I sign an FJO or after I start my first day at work?