Avoid using datetime.min

Using strftime on datetime.min on Python 2.7 throws an Exception:
year=1 is before 1900; the datetime strftime() methods require year >= 1900
This should not happen, could have an other root cause and is OK with Python 3

Target issue:
https://www.synology-forum.de/showthread.html?37898-tvstreamrecord-Aufzeichnen-von-HTTP-Streams&p=866321&viewfull=1#post866321