The clock starts when he is picked up and taken into custody, not when the paperwork is signed.
Signing the revocation papers on Monday set the legal process in motion, but time served does not begin until physical custody begins. The moment law enforcement or transport takes him into custody is day one, and that day counts as a full day regardless of what time it happens. If they pick him up at 11:55 at night, that entire calendar day counts as a day served. The system counts calendar days, not hours, which means the timing of the actual pickup can matter more than most people realize.
From pickup to arrival at the designated facility, all of that time in custody counts too. Whether he spends a night in a county jail waiting for transport, moves through a classification center, or gets held briefly at a transfer facility, every day in custody from the moment of pickup forward is credited toward his sentence.
The signed paperwork is the legal trigger, but it is the physical custody that starts the sentence clock. If there is any delay between signing and pickup, those days in between do not count. Once he is in their hands, the count begins.