Date format issue on Mac

[cpashk]

I believe this use of DATETIME_FMT and here is causing errors. Seems that some C code backing strftime doesn't behave well across all platforms. I was able to get this working locally using the DATETIME_FMT_MAC instead of DATETIME_FMT. Current singer-python has a DATETIME_FMT_SAFE but it looks like that's the same format, just a more generic name.