What does it mean.....

Not true. At least not in every case. The FAA most definitely uses 00:00 to signify midnight within the national airspace system.

 

Mardet and jgremlin are both correct but 00:00 in a Swift pplan means no appointment has been set. That can be a bad thing when the dispatch shows an impossible delivery time. Mac22 is your friend; do your loaded call and the mac22 at the same time.
Frank

 

Yea, if it's 00:00 and a live unload .. lookout!
Can mean it's one of those jerky places with militant rules
about appointment times and what not.
So of course you'll get a message several hours later that
the appt. is for 2 hours before you could ever get there.
Mac 22 time, reason: no appt. time given on preplan.

Of course once you do get there, the final will ##### that you are late,
then take however (super) long they feel like to unload you as a 'favor'.
And probably forget about detention pay...
You didn't 'make your appointment on time'!