The sentence is 例えば外食時、定食屋さんでとても美味しいご飯をたべたとする。
I've looked at some other answered questions which explain how とする, when appended to a verb expresses quitting an old action and starting a new one. However I can't understand it in this case, because it seems odd that it would be used in the past tense.
Does it take on a different meaning if the verb is in the past tense?