These are Unix timestamps. They represent the number of seconds that have passed since the Unix Epoch, which occurred at 12:00 am UTC on January 1, 1970.
You can convert a human-readable date to / from a Unix timestamp using Wolfram Alpha. Example queries:
"convert January 15, 2005 14:56 PST to unix timestamp"
"convert unix timestamp 1105829760"
The end of the range is the highest possible date represented by as a timestamp.
convert unix timestamp 253402300799 Friday, 31 Dec 9999 23:59:59 GMT