Fix for date/time format following nagios3 documentation

Hi I just made a small change following nagios3 docs:

#       us              (MM-DD-YYYY HH:MM:SS)
#       euro            (DD-MM-YYYY HH:MM:SS)
#       iso8601         (YYYY-MM-DD HH:MM:SS)
#       strict-iso8601  (YYYY-MM-DDTHH:MM:SS)


if none of us|euro|iso8601 is passed when calling new it defaults to strict-iso8601 (for version >= 3).

Cheers!