Is it just a coincidence that the Jewish war in AD 70 lasted 3 1/2 years (1290 days)
Nope. It was a type of what'll happen again big time. I wonder how many times Christ had an Olivet discourse with His disciples?
The parallel accounts in Matt 24 and Luke 21 show that additional questions were asked as well.
1. When will these things be?
2. What is the sign of your coming...?
3.... And the end of the age?
I notice that His answer covered all these points from 70 AD all the way to the end when He returns. The called had better be watching.