shitatoshite and surutoshite

I have a question regarding the use of 「したとして」VS「するとして」in daily conversations. Are they completely interchangeable, or is one more grammatically correct compared to the other? I understand that した by itself is the past tense of する, but what if when speaking hypothetically? For example:

例えば今日世界が終わったとして
例えば今日世界が終わるとして

Excuse me if these phrases came out as awkward, but would one translate the former as "If the world were to have ended today" and the latter as "If the world were to end today?" I'm not sure of the difference in English myself, but maybe one of the above phrases just makes no sense. Thank you!

If you say "owatta to shite," it's already ended. or you are assuming a situation it's already ended (because of the "-ta" form).
If you say "owaru to shite," then it hasn't ended but it will today.

So if you are starting sentences with those "if" clauses, you might continue on to say something like:

例えば今日世界が終わったとして、何が残っていると思う？(Suppose the world has ended today, what do you think would be remaining?)

例えば今日世界が終るとして、何をしたいと思う？(Suppose the world is going to end today, what do you think you want to do?)