Julian Date Converter

Convert Between Dates and Julian Numbers

What this tool actually does

This calculator makes it easy to switch back and forth between regular calendar dates and something called Julian dates. You can either enter a standard date and time (like June 11, 2025, at 6:30 PM), and it’ll show you the Julian version of it—or do the reverse: plug in a Julian number and get the corresponding calendar date back out.

It handles different types of Julian formats too, like Astronomical Julian Date, Julian Day of Year, and Modified Julian Date. Whether you’re working on astronomy data, writing software that uses date offsets, or just curious about timekeeping systems, this tool covers the bases.

Why someone might actually want to use this

Julian dates might sound like something you only need in a telescope lab, but they pop up in more places than you’d expect. Scientific research, satellite tracking, and even weather models use them because they’re clean and continuous—no worrying about leap years or timezone shifts.

On the flip side, if you’ve been handed a Julian number in a spreadsheet or system log and have no clue what it represents, this converter gives you an exact calendar match, including time and timezone handling. No math, no memorizing formulas.

How to use it: quick walkthrough

Step 1: Choose your direction

You start by selecting what kind of conversion you need. Pick between:

• Date to Julian: Turn a calendar date into a Julian number
• Julian to Date: Go from Julian back to a regular date

Step 2: Enter your inputs

For a standard date, just use the calendar and time pickers. There’s also a dropdown for timezone—handy if you’re not working in UTC.

If you’re converting from a Julian number, you’ll enter it manually. There’s a type selector too, which matters more than you might think. Not all Julian numbers are the same kind:

• Astronomical Julian Date: Long decimal starting from 4713 BCE
• Julian Day of Year: Just the day number in the current year (like 75 for March 15th in a leap year)
• Modified Julian Date: A shorter version used in many systems today

Step 3: Hit Convert

One tap on the “Convert Julian Date” button and everything updates. The result panel shows your main answer—plus bonus info like the day of the week, whether it’s a leap year, and both Gregorian and Julian breakdowns. You even get the modified Julian version and day-of-year just for good measure.

Tips, quirks, and features tucked in the interface

Live time updates

The calculator quietly keeps track of the current time and Julian date in the corner. It updates every second, which is useful if you’re trying to line something up to the exact moment.

Time format toggle

Not a fan of AM/PM? There’s a toggle right inside the tool to flip between 12-hour and 24-hour time formats. It even affects how your results are displayed.

Timezone offset quirks

The timezone support here is simplified—some daylight saving changes aren’t fully covered. It still works well for general use but keep that in mind if you’re after scientific-grade precision.

It calculates on its own

As soon as you change a date, time, Julian number, or conversion method, the tool recalculates instantly. You don’t have to press anything unless you want to.

Visual cues for success

When you hit convert, the result area gently highlights—just enough to let you know the output has refreshed without shouting about it.

Stay on track with your timeline, one conversion at a time

Whether you're double-checking an old observatory log, syncing system clocks, or just plain curious how dates line up across time systems, this converter gives you what you need in one place. No formulas to memorize. No flipping between calendars. Just clear answers, fast.