Some actions and effects in combat are timed - they last for a set amount of time, usually measured in rounds or minutes. Timed effects can also be effects that take place repeatedly in consistent intervals, such as every minute, every round, or at the beginning or end of a creature's turn.
An effect might state that it occurs at the beginning or end of a combatant's turn, or of the current round.
Beginning-of-turn effects take place when the affected combatant's turn starts and must be evaluated before the combatant can declare any actions.
End-of-turn effects take place when the affected combatant's turn ends; that is, as soon as the creature has completed an action that leaves them with no remaining action frames, ends its turn early, or starts an action that will take until their next turn to complete. These effects must be evaluated before the next combatant's turn can begin.
End-of-round effects take place after the end of the last turn in the initiative order, after any end-of-turn effects on the last turn of the round and before any beginning-of-turn effects on the next turn. Time-based effects that don't originate from any combatant, such as an environmental hazard like a rising river or an effect being caused by a creature not currently in combat, typically happen as end-of-round effects but may become end-of-turn effects for that creature if they enter combat.
Any beginning- or end-of-turn effects tied to a creature that continue to apply after the creature dies or leaves combat move to the next turn in the initiative order.
An effect might state that it lasts for a set number of rounds, or a set amount of in-game time measured in minutes, hours, days, or a fictional unit of time unique to the game world. This is a Timed Effect.
Round-based Timed Effects that begin during a combatant's turn have a Round counter that starts on the specified number, and decreases by 1 as a beginning-of-turn effect for the combatant whose turn it was when the effect started. As soon as the round counter is decreased to 0, the effect no longer applies. For example, if a 2-round effect started on the second combatant's turn, its round counter would start at 2. It would decrease to 1 at the beginning of that combatant's next turn, then to 0 at the next turn after that, at which point it would end.
Effects that last minutes, hours, or a longer time are less exact in their timing. For the purposes of time conversion, one round is equal to six seconds of in-game time (ten rounds equals one minute.)
For timed effects that are active outside of combat, it's up to the GM to manage the time and determine when the effect ends.
Some abilities and effects might state that they can be used a set number of times per day. These are Daily Effects.
Daily effects typically reset when the creature they apply to finishes a full rest, although the GM can reset them when a day has passed even if the party hasn't fully rested. It may therefore be possible for a creature to use an ability with a daily allowance right before a rest, and then use it again immediately after the rest, even though a full day hasn't passed. This is distinct from a timed effect lasting a set number of days, which would last for exactly that amount of time regardless of rest timing.