Represent timestamps using numbers

[msiebuhr]

The timetamps should be numbers instead of string (64 bit floats vs. string-representation).

I'm aware that floats can't represent all integer values, but as a 64bit int can represent all integers up to 2^53th. (Using UNIX time w. millisecond resolution, that results in October 12, 287396 - which I guess means it will work for a forseeable future.)