[pdftex] pdfcreationdate format(s)

[Akira Kakuto]

Dear Thierry,

> D:20171205171912+01'00'
> D:20171205162354Z

I think the former is a local time, and the
latter is a universal time.

For example, if SOURCE_DATE_EPOCH is suitably defined, the
universal time is used.

I think that the variable SOURCE_DATE_EPOCH in
pdfTeX etc. is introduced in 2016.

Thanks,
Akira