Is there a past tense for the word diary?

The word "diary" is a noun, not a verb. Nouns do not have tenses.

If you are referring to the action of writing in a diary, the past tense would be "wrote in a diary" or "kept a diary".

For example:

* Present: I write in a diary every day.

* Past: I wrote in a diary every day when I was younger.

* Present: She keeps a diary.

* Past: She kept a diary for years.