I would recommend to remove time-based status effects on the start of the turn of the player who applied them. Why? Because it gives that player the opportunity to uphold status effects indefinitely by reapplying them immediately when they time out (assuming they got the resources to do that, of course).

Other options lead to weird situations you probably want to avoid.

• At the end of turn of the player who applied them or at the start of the player who owns the unit: The current player might find themselves in a situation where the UI shows them a ton of attack, movement and action denial effects on various enemy units. But those are all meaningless, because they are going to be over when the player who owns the unit gets their turn. The UI is misinforming the player. You generally don't want that.
• At the end of turn of the player who owns the unit: In a 2 player combat, this is more or less equivalent to the start of the applying player (assuming no edge cases from order of operations that would require me to make too many unfound assumptions about your game mechanics). But consider a battle with 3 or more factions. Now whether your ally can benefit from the debuffs you apply depends on move order. When the ally has their turn before the enemy, then you can support them by debuffing enemy units for 1 turn. When the enemy has their turn first, you can't. Do you really want that kind of metagaming in multi-faction battles?

I would recommend to remove time-based status effects on the start of the turn of the player who applied them. Why? Because it gives that player the opportunity to uphold status effects indefinitely by reapplying them immediately when they time out (assuming they got the resources to do that, of course).

Other options lead to weird situations you probably want to avoid.

• At the end of turn of the player who applied them or at the start of the player who owns the unit: The current player might find themselves in a situation where the UI shows them a ton of attack, movement and action denial effects on various enemy units. But those are all meaningless, because they are going to be over when the player who owns the unit gets their turn. The UI is misinforming the player. You generally don't want that.
• At the end of turn of the player who owns the unit: In a 2 player combat, this is more or less equivalent to the start of the applying player (assuming no edge cases from order of operations that would require me to make too many unfound assumptions about your game mechanics). But consider a battle with 3 or more factions. Now whether your ally can benefit from the debuffs you apply depends on move order. When the ally has their turn before the enemy, then you can support them by debuffing enemy units for 1 turn. When the enemy has their turn first, you can't. Do you really want that kind of metagaming in multi-faction battles?

Now about trigger effects of DOT effects like poison. This is a question with no right answer. It depends on how you want the game to play:

• Beginning vs. end of player who owns the affected unit: Do you want a poisoned unit that is destined to die from poison damage this turn to always be able to perform one last action? Do you want the owning player to have an opportunity to heal poison before it even did its first damage?
• End of turn of applying player vs. somewhere on turn of owning player: Do you want any 3rd factions taking their turn in between to be able to interact with units that are going to die from poision damage anyway?

I would recommend to remove time-based status effects on the start of the turn of the player who applied them. Why? Because it gives that player the opportunity to uphold status effects indefinitely by reapplying them immediately when they time out (assuming they got the resources to do that, of course).

I would recommend to remove time-based status effects on the start of the turn of the player who applied them. Why? Because it gives that player the opportunity to uphold status effects indefinitely by reapplying them immediately when they time out (assuming they got the resources to do that, of course).

Other options lead to weird situations you probably want to avoid.

• At the end of turn of the player who applied them or at the start of the player who owns the unit: The current player might find themselves in a situation where the UI shows them a ton of attack, movement and action denial effects on various enemy units. But those are all meaningless, because they are going to be over when the player who owns the unit gets their turn. The UI is misinforming the player. You generally don't want that.
• At the end of turn of the player who owns the unit: In a 2 player combat, this is more or less equivalent to the start of the applying player (assuming no edge cases from order of operations that would require me to make too many unfound assumptions about your game mechanics). But consider a battle with 3 or more factions. Now whether your ally can benefit from the debuffs you apply depends on move order. When the ally has their turn before the enemy, then you can support them by debuffing enemy units for 1 turn. When the enemy has their turn first, you can't. Do you really want that kind of metagaming in multi-faction battles?