# Julian Day

**Julian day** is a count of days since the beginning of the Julian period, used to calculating days between any two events.

The calendar, with twelve months of the year, each with varying number of days, and February with 28 or twenty nine days in leap years, makes it difficult and complicated to calculate the time between any two events. Joseph Scaliger proposed that a Julian Period be used to time events, beginning at a time sufficiently far in the past as to precede dates in recorded history. The Julian period begins Monday, 1 January 4713 B.C.

**Julian Day** counts the number of days from the beginning of the Julian period allowing the calculating of event times. **Julian Day Number** is an integer number assigned to any one day, the JD beginning at noon. The **Julian Date** is an instant given as a decimal part of the day since the previous noon.

The **Modified Julian Date** is the number of days (and fractional days) elapsed since midnight GMT on 17 November 1858. This is 2400000.5 smaller than the standard Julian Date.

In Orbiter it is used to define the current time in scenario files; it is also displayed in the HUD.

## Calculations

**To convert a Gregorian calendar date to Julian Day Number, used for dates on or after 15 October 1582 for Catholic countries, other countries changed on other dates):**

JDN = (1461 × (Y + 4800 + (M − 14)/12))/4 +(367 × (M − 2 − 12 × ((M − 14)/12)))/12 − (3 × ((Y + 4900 + (M - 14)/12)/100))/4 + D − 32075^{[1]}

where

- Y = year
- M = month (1-12)
- D = day of month (1-31)

**To convert a Julian calendar date to Julian Day Number, used for dates on or before 4 October 1582 for Catholic countries, other countries changed on other dates):**

JDN = 367 × Y − (7 × (Y + 5001 + (M − 9)/7))/4 + (275 × M)/9 + D + 1729777

where

- Y = year
- M = month (1-12)
- D = day of month (1-31)

**To convert Julian Day Number to Julian Date, given time of day:**
JD = JDN + (h - 12)/24 + m/1440 + s/86400

where

- h = hour
- m = minute
- s = second

**To convert Julian Date to Modified Julian Date as used by Orbiter:**
MJD is used since the beginning of the space program.

MJD = JD - 2400000.5