It's a format called "Date-Time Group" (DTG). Used in military, I think it dates back to teletypes, and is better to say over radio than modern ISO formats.

You'd say "Valid from the thirteenth at two zero five three zulu" over radio.

Airplane software is some of the oldest on the planet, pre-dating anything that would be considered a best practice. When you see something weird in the space, assume it is because someone in the 1960s thought using more than two characters world be a profligate waste of space.

Unrolling these decisions is a monumental task given the decades of hardware already deployed in the field that will only be upgraded when the current unit fails.

My guess at parsing: in effect from 8:53pm UTC on the 13th to 4am UTC on the 14th.

Friday the 13th at 08:53 PM UTC

UTC