Julian Date Converter

What this tool does

Julian Date Converter converts a Gregorian calendar date into its Julian Day Number (days counted continuously since noon UTC, 1 January 4713 BCE) and Modified Julian Date, or converts a Julian Day Number back into its corresponding Gregorian calendar date.

Astronomy, satellite tracking and GPS systems need a way to express dates that counts continuously without the irregular month lengths and leap-year exceptions of the civil calendar, which is exactly what the Julian Day Number system provides: a single ever-increasing integer counting days since a fixed reference point far in the past.

This converter handles both directions: turning a familiar Gregorian calendar date into its Julian Day Number (and the shorter Modified Julian Date figure used in many satellite systems), or taking a Julian Day Number and finding the corresponding Gregorian calendar date.

How to use Julian Date Converter

1. Choose a Direction: Gregorian date to JDN, or JDN to Gregorian date.
2. For the forward direction, enter the Date.
3. For the reverse direction, enter the Julian Day Number.
4. Read the resulting Julian Day Number (and Modified Julian Date), or the resulting Gregorian date.

Tips for better results

• The Julian Day Number counts from noon UTC, not midnight, which is why astronomical references sometimes differ by a fraction of a day from a naive midnight-based day count.
• Modified Julian Date (JDN minus 2400000.5) shifts the day boundary to midnight and uses smaller, more manageable numbers, which is why it is the preferred figure in many satellite and GPS systems.
• The Julian Day Number system is unrelated to the historical Julian calendar used before the Gregorian reform; it is purely a continuous day-counting convention, despite the similar name.
• This converter always works in the modern Gregorian calendar; it does not model the historical Julian calendar that was actually in civil use before 1582.

Common use cases

• Converting an astronomical observation date into its Julian Day Number for a calculation or reference lookup.
• Converting a satellite or GPS system’s Modified Julian Date back into a readable calendar date.
• Cross-referencing a date between an astronomy textbook or dataset and a normal calendar date.
• Understanding the relationship between a civil calendar date and the continuous day-counting conventions used in science and engineering.

Common mistakes to avoid

• Confusing the Julian Day Number system with the historical Julian calendar; despite the shared name, they are unrelated concepts, one a continuous day count, the other a historical civil calendar.
• Forgetting that the Julian Day Number changes at noon UTC, not midnight, which can cause an off-by-one confusion when comparing against a midnight-based date.
• Mixing up Julian Day Number and Modified Julian Date; they differ by exactly 2400000.5, and using the wrong one in a formula that expects the other gives a drastically wrong result.