The Constitutional Court on Wednesday set Sept 30 for its ruling on when Gen Prayut Chan-o-cha's eight-year tenure as prime minister began and ends.
Gen Prayut, who seized power in a 2014 coup before formally becoming prime minister, is currently suspended while the court deliberates on the case filed by the opposition. His deputy, Gen Prawit Wongsuwan, is serving as caretaker premier.
On Aug 24, the Constitutional Court suspended Gen Prayut from his leadership position as the opposition asked it to rule on his eight-year term.
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Section 158 of the 2017 Constitution says: "The Prime Minister shall not hold office for more than eight years in total, whether or not holding consecutive terms. However, it shall not include the period during which the Prime Minister carries out duties after vacating office."
That leaves Gen Prayut's period as prime minister open to debate.
The opposition and opponents of the 68-year-old leader argued that his term should end on Aug 24, because he was installed as prime minister on Aug 24, 2014, under the provisional constitution enforced after he led the coup on May 22, 2014.
Others argue that his first day in office was April 6, 2017, when the present charter was enacted, while Gen Prayut's supporters say his Day One was June 9, 2019, when he became the prime minister after the general election in March under the 2017 constitution.
The suspended prime minister now works as the defence minister.
Thailand is under pressure to end the uncertainty over its leadership before world leaders arrive in Bangkok for an Asia-Pacific Economic Cooperation (Apec) summit in November.