What is the standard date format used in aviation?
I'm not sure if there is really an industry wide standard for the date format, but at least the ICAO (International Civil Aviation Organization) uses the YYMMDD date format for flight plans:

DOF/ The date of flight departure in a six figure format (YYMMDD, where YY equals the year, MM equals the month and DD equals the day).

(FAA on ICAO flight plans)
Other flight plans may use different formats however. See e.g. the following example of a LIDO flight plan (Lufthansa Systems format):

(image source: SimBrief.com)