This is quite an interesting topic to explore, and one which I spent a while researching as well.
Assuming Apple uses Unix Time and measures time in 64-bits (which your iPhone 6s processor is capable of), then the highest date it would theoretically be able to store would be UTC 15:30:08 on Sunday, 4 December, 292,277,026,596 AD.
There are some 32-Bit systems that will suffer what is known as the Year 2038 problem after 03:14:07 UTC on 19 January 2038.
Obviously, this doesn't take into account any of the other limitations of the device, hardware or software. From the looks of SravanKrA's screenshot, software limitations have probably been put in place to limit Calendar to the year 10, 000.
Nonetheless, it's certainly an interesting topic to explore from a computer programming perspective.
There's a lot of great examples of date and timekeeping bugs in computer programming over at https://en.wikipedia.org/wiki/Time_formatting_and_storage_bugs