__TIMESTAMP_UNIXEPOCH__ instead of useless __TIMESTAMP__?

[Timothee Cour]

__TIMESTAMP__ is pretty useless:
`string literal of the date and time of compilation "www mmm dd hh:mm:ss yyyy"`
eg:Wed Jan 24 11:03:56 2018
which is a weird non-standard format not understood by std.datetime.
__DATE__ and __TIME__ are also pretty useless.

Could we have __TIMESTAMP_UNIXEPOCH__ (or perhaps
__TIMESTAMP_SYSTIME__ to get a SysTime) ?

from that, users can convert to whatever format they want.