A day will be 24 hours, 39 minutes and 35.244 SI seconds long
A day will be 24h×60m×60s using a "mars second" equal to 1.0275 SI seconds.
Other
A day will be 24h×60m×60s where a "second" is a unit of angle rather than time, equal to 1/86400 of a revolution w.r.t the sun (i.e. the equivalent of Earth's UT1, this is called "Coordinated Mars Time")
A day will be 24h×60m×60s using SI seconds (and will drift with respect to mean solar time by ~40 minutes a day)
Time of day will be an incrementing number of SI seconds from 0 to 88775.244 (possibly rounded to an integer, with leap-seconds every 4 days or whatever, or no leap seconds and allowing it to drift)
