What would you say to a so-called Christian who claims even almighty God's day of Armageddon might happen before his set date, due to ?

We have nothing to learn from the so-called Christians, in fact, any religion defines itself as Christian? NO!

We have the word of God described in the book of revelation, first Armageddon will come, there is no date written, surely God will reveal it in future years through his True prophets. After Armageddon, it does not mean that the life of mankind will cease, there will still be years, but in the end, human life on this earth will also end. Then, we all know, don't we? God's judgment will take place for all human beings!

I say to them:  prove your god is real.

And then, I watch them have a meltdown, 

poop their pants, and run home crying to mommy.

Armageddon will happen according to prophetic events at God's appointed time.

Just giggle.

The jehovah's witnesses have done this continuously throughout their cult's history.

You know what the bible says about false prophets and their followers.

I would not say anything to them because most of those who said the very same things that you are saying here, currently occupy graves and have nothing to show for what they proclaimed.

I would say that the knowledgeable people understand that the events in Revelation happened in the first century.