According to standing global treaty organizations specifications, 'What is the official universal time system of Mars?'
It's not needed, so no "global treaty organizations" would be interested in spending even a minute thinking about it.
In the meantime (so to speak), each landed mission on Mars plans its operations to the Local Mean Solar Time, which is defined by a fixed longitude related to the targeted or actual landing site.
There is no official universal time system for Mars.
However, NASA has proposed an analog for Universal Time on Earth, that they refer to as Coordinated Mars Time, or MTC:
The prime meridian of Mars is defined by the location of the crater Airy-0 (De Vaucoulers et al., 1973), named in honor of the British astronomer George Biddel Airy, who built the telescope at Greenwich whose location defines the prime meridian on Earth. Although it might be tempting to refer to "standard" time on the Mars prime meridian as "Airy Mean Time" (AMT) in analogy to Earth's "Greenwich Mean Time" (GMT), the latter term has been supplanted by Coordinated Universal Time (UTC) in international timekeeping services. Mars24 therefore refers to the Mean Solar Time on the Mars prime meridian as Coordinated Mars Time, or MTC, by analogy to the terrestrial UTC.
This system is referred to in the Wikipedia article on Martian timekeeping.