Search notes:

UTC

UTC stands for Coordinated Universal Time (or Universal Time Coordinated).

UTC is a successor of and conceptually different from GMT. Nevertheless, UTC is sometimes referred to as GMT.

UTC is independent of time zone values.

Each time zone is expressed as a positibe or negative offset from the UTC.

UT, TT, Delta T

Universal Time (UT) is a time scale based on the Earth's rotation

In contrast, Terrestrial Time (TT) is a (theoretical) uniform time scale.