There is no other context. "t" is time. In a paragraph, I want to write "t is a time in the past", but it is bit too long. So I ask if "t is a past time" or "t is a time of past" is right in that case.
I google it and find "past time" may mean something else, such as something that should have already been done or already occurred, which is not what I want it to mean. I just want "a past time" to mean a time in the past.