Julian Date Calculator is an iPhone app by CosmoWerx, aka Milan Battelino. Currently at version 1.3, it costs 99 cents in the App Store. As the name indicates, this app calculates Julian Dates, which count the days since the beginning of the Julian Period, 4713 BCE, including the decimal time from noon UTC (GMT). To convert any (Coordinated) Universal Time and calendar date to a Julian Date, first enter the time on the picker and tap the date to change it, or vice versa. You may also tap the "UTC Now!" button to get the current UTC time and Julian Date, which generates a loud "ping!" sound.. However, you have to convert between local and UTC time yourself.
To convert a Julian Date to a calendar date and UTC time, simply tap on the Julian Date and a number pad will pop up, allowing you to edit the field, then tap the "Done" button to convert. You can also select and copy the Julian Date to paste into another app.
There is a second field under the Julian Date, which displays an additional decimal date, which you can select by clicking on the "i". Additional date options available include:
- Modified Julian Date
- Reduced Julian Date
- Truncated Julian Date (NASA)
- Truncated Julian Date (NIST)
- Dublin Julian Date
- ANSI Date
- Rata Die
- Unix Time
- RAMSES Time
Selecting any of these will display a description of what they are. The last two are second counts, rather than day counts. RAMSES Time is new to me, and is explained as the number of seconds since the beginning of 2000, MJD 51544.0, developed by the Swedish Space Corporation.
There is also an iPad version, named Julian Date Converter, for $1.99, which shows all the decimal dates and times at once, including GPS Time.