Currently I am building a Bash based toolkit to troubleshoot and analyze SSL/TLS certificates and certificate requests. I ran into a confusing problem and likewise interesting problem during the development:

date -j -f '%b %d %T %Y %Z' 'Aug  7 07:09:42 2014 GMT' +%s
date -j -f '%b %d %T %Y %Z' 'Oct 27 17:35:57 2013 GMT' +%s
Failed conversion of ``Oct 27 17:35:57 2013 GMT'' using format ``%b %d %T %Y %Z''
date: illegal time format
usage: date [-jnu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ...
            [-f fmt date | [[[mm]dd]HH]MM[[cc]yy][.ss]] [+format]

As you can see the first statement returned our timestamp without any problem, but the second one fails with a date: illegal time format message even though we use the same command options/syntax. Why is that? The answer is rather simple: month and day representations have to be your in system's nationality/language!

So if you change the Oct to Okt (I am running a german OS X), the conversion works without any problems.

How to solve this?

Either you change your date informations to the systems nationality, or you do this simple trick:

LANG=C date -j -f '%b %d %T %Y %Z' 'Oct 27 17:35:57 2013 GMT' +%s

Putting a LANG=C in front of the command tells date to act normal and not OS X-like so you don't run into this problem as well.

Credits to drkshadow out of the #bash who assisted me here.