JD days may have decimal fractions which correspond to the time of day. The Julian day begins at noon, and the decimal fraction measures fractional days until the beginning of the next day at noon.
For instance, Julian Day 2453179.00000 is June 22, 2004, at 12:00pm (noon).
One hour later, it's 2453179.04167
At 2453179.20833 I'll have dinner, and
at 2453179.45833, it's time for the evening news.
After a good night of sleep, my alarm will go off at 2453179.83333,
then at noon on June 23, a new Julian Day begins at 2453180.
To use these functions with fractional days, strip the fractional part with floor(), and apply the function to the integer part.
Then add 12 hours, bringing you to noon of that day. That is the actual time returned by JDToGregorian().
Then add the fractional part of the day, by multiplying the decimal part of the Julian Day by (24*60*60) seconds. This may take you forward or backward to a different Gregorian calendar date.