But this is not the verdict Gen Prayuth's supporters had been hoping for. They have been proposing June 2019 as the start date, when he was re-appointed prime minister following the first post-coup parliamentary election earlier that year.
This is important because Gen Prayuth has made it clear he would like to contest the next election, expected early in 2023, aiming to become Thailand's longest-serving prime minister - not bad for an army commander who said he was only taking power temporarily.
The Constitutional Court ruling means he has just two-and-a-half years left until he hits the term limit, making him less appealing as a prime ministerial candidate to conservative parties.