Difference between "on that time" and "at that time"

[Littlewonder]

hello.
can anyone explain to me what's the difference between those two sentences?
on that time and at that time..
how and when can we use it ?
example like...class will be held on that time / at that time ?
thank you for your concern.

[Alan]

Hi,

I wouldn't say: On that time. The preferred expression is: At that time. You could say: On that occasion.

Alan
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[Littlewonder]

thank you for your reply.