Paused Timelines

It is difficult for datapacks to change environment attributes on demand. This problem can be solved easily with paused timelines. Each timeline can have a parameter indicating whether it is running by default. After that, you can pause or resume specific timelines with commands, or set a timeline to a specific position.

Even better, timelines could be different for each player (e.g. timeline position (and paused state) is stored in a playerdata .nbt file).

For example:
/timeline (timeline ID) (player selector) set running true
/timeline minecraft:day @a set ticks 0
/execute as @a store result score @s scoreboard run timeline (timeline ID) @s get time