Variable star observations reported to the AAVSO must be expressed either in terms of Universal Time (UT) or Julian Day (JD) and the decimal part of the day given in Greenwich Mean Astronomical Time (GMAT).

JD is the standard unit of time used by astronomers because it is convenient and unambiguous. Here are the advantages:

• The astronomical day runs from noon to noon so that you don’t have to change calendar dates in the middle of the night.
• A single number represents days, months, years, hours, and minutes.
• Data on the same star from people observing anywhere in the world can be compared easily since they are all relative to the same time zone; that of the prime meridian in Greenwich, England.

AAVSO provide an annual JD calendar generator and a JD-UT converter.

https://www.aavso.org/jd-calculator