Spacecraft Event Time is the time an event occurs in relation to a spacecraft. Since it takes time for a radio transmission to reach the spacecraft from the earth, the usual operation of a spacecraft is done via an uploaded commanding script containing SCET markers to ensure a certain timeline of events. Real-time commanding of spacecraft is done rather rarely, mostly in case of time constraints during an emergency event like going into safe mode

Safe mode (spacecraft)

Safe mode is an operating mode of a modern spacecraft during which all non-essential systems are shut down and only essential functions such as thermal management, radio reception and attitude control are active.-Triggering events:...

 for an earth-pointed CME

Coronal mass ejection

A coronal mass ejection is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space....

 of the sun.
The Spacecraft Event Time of a singular event on the spacecraft can also be defined as equal to the ERT (Earth-Received Time) minus the OWLT (One-Way Light Time), or as TRM (transmission time) plus OWLT.

Spacecraft Event Time in UTC

Coordinated Universal Time

Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

is also known as Orbiter UTC, and Earth-received time as Ground UTC.