Julian day
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, 1 January 4713 BC in the Julian calendar. That noon-to-noon day is defined as Julian day 0. Thus all JDNs that are evenly divisible by 7 are Mondays. Negative values may also be used; although, those predate recorded history.