Simplify the following text for a Grade 2 native English-speaking person.
Original: The Julian day or Julian day number ( JDN ) is the number of days that have passed since the initial epoch defined as noon Universal Time ( UT ) Monday , January 1 , 4713 BC in the Julian calendar . That noon-to-noon day is counted as Julian day 0 .
Simplified: Now , at the Julian day number is .