International Astronomical Union

The International Astronomical Union (IAU) has been using decimal time in its telegraphed circulars for over a century. Following the recommendations of the IAU, the dates of observations, or of astronomical events, should be written in the order of year, month, day, so that in principle it can be appended by the decimal fraction of a day. An example decimal time would be 2023 Nov. 3.1234 UT, where .1234 is the time of day represented as a fraction. All times are Universal Time (GMT). This fraction is identical to MJD, but with a Gregorian calendar date. 

 For an actual example, see IAU circular