from time to time

On or near the same date (in the past or in the future). / On this occasion (as opposed to last time or next time).

Never.

It was once true (that)...; there was a time when...

When (some future event has come to pass).

Punctually or according to the schedule. / By installments, payments over a period of time. / (chess) By a player's running out of time.

Punctual or according to schedule.

(tennis) Reminder by the umpire for the players to continue playing after their pause. / The umpire's call in prizefights, etc.

(physics, usually uncountable) A dimension of spacetime with the opposite metric signature to space dimensions; the fourth dimension.

To measure or record the time, duration, or rate of. / To choose when something begins or how long it lasts. / (obsolete) To keep or beat time; to proceed or move in time.