Convert Gregorian calendar dates into Julian Day (JD), Modified Julian Date (MJD), Day of Year (DOY), and 5-digit manufacturing date codes.
The Julian Day (JD) system, devised by French scholar Joseph Justus Scaliger in 1583, provides a continuous tally of solar days and fractional days from an origin epoch set on January 1, 4713 BC. Unlike Gregorian dates, Julian days eliminate irregularities caused by leap years, differing month lengths, and daylight saving shifts—making them the gold standard in astronomy, satellite orbit tracking, and geodesy.
Modified Julian Date (MJD): Astronomers frequently employ the Modified Julian Date ($MJD = JD - 2400000.5$), which sets the zero reference point to midnight UTC on November 17, 1858. This reduces the magnitude of numbers and aligns day rollovers with midnight rather than noon.
Commercial & Manufacturing Codes (YYDDD): In supply chain logistics and batch tracking, companies often print a 5-digit "Julian code" where the first two digits designate the production year and the final three digits indicate the sequential Day of the Year (DOY 001 to 366).
The Julian Calendar is the historical solar calendar introduced by Julius Caesar in 46 BC. In contrast, the Julian Date is a continuous mathematical count of days since 4713 BC, unrelated to the historical Roman calendar.
Astronomers historically adopted noon UTC so that nighttime astronomical observations would fall within a single calendar day without changing dates at midnight.
The first two digits indicate the year (e.g., 26 for 2026), and the remaining three digits indicate the day of that year (e.g., 045 represents the 45th day of the year, February 14th).
Unix time counts seconds elapsed since January 1, 1970 UTC (JD 2440587.5). The Julian day number can be converted directly to Unix timestamps by subtracting the epoch difference.
