Which sounds like apologetics at their best (the best they can muster anyway) to me. The "atheists" said "not compelling", so the Christians went back to their numbers and jiggered them some more, found some justifications, and (again, I insist these are absolutely estimates - are dates really used in The Bible? If so, what are these dates?) ultimately pushed the results back into the "prophecy fulfilled" zone. You've basically just described this very process with your little paraphrasing above.
Are dates really used in the Bible? The prophecies and dates given are to precise days in the Hebrew festival calendar. Dates are also alluded to like "in the X year of Y's reign over Babylon", etc.
1) The 430 and 70 prophecies are given
for Israel's sin.
2) Atheists noted that following the 70 in Babylon
for Israel's sin, the Jews should have had a Jewish-controlled Holy Land after only 360 years, and that the diaspora went far longer.
3) The Christian could do nothing, really, before someone noticed that in another passage it says
because of Israel's sin the punishment will be multiplied times 7.
4) Someone plugged in 7 multiplied by 360 and arrived at May 1948 and said WHOA! WHOA! WHOA WHOA!
I'd settle for one "Whoa!" from you. Know this, atheists have only said this to me:
1) Let's question the archaeology verifying the original dates and move the prophecy around, like to '45 or '51, invalidating it
2) Whoa is right--they got super-lucky there!
3) The Jews could have made Israel Jewish earlier, but self-fulfilled the prophecy (ridiculous on its face)
4) Even if the prophecy is right, it's all a magnificent coincidence, and I need more evidence that the Bible predicts the future, showing God is real
5) "You're just massaging together the two prophecies, you know, the only two about the length of the diaspora, the one that indicates the 430/360 and the one that multiples it by 7, which is kind of vague"
Do you have a 6) for me or not? I'm VERY open to a 6) stronger than the very weak 5 given above by your colleagues.
I didn't know these prophecies before I trusted Jesus for salvation, but there are others, if you're intrigued.