I often find myself using Julian days as a simple method to represent dates in my code. It’s nice and easy, because every day is simply an integer (the number of days since the beginning of the year) and any time during the day can be represented as a floating point number (the fraction of that day which has elapsed at that point). Furthermore, lots of satellite imagery is provided with the dates specified as Julian days – for example, MODIS data filenames include the year and then the Julian day.
It’s fairly easy to convert a standard date (eg. 24th March) to the Julian day in most programming languages (there will either be a library function to do it, or it’s fairly simple to write one yourself) – but it’s not that easy to do in your head. So, I have created a Julian Day calendar poster:
It’s a very simple table that lets you choose the month and day, and get the Julian Day number. Just make sure you remember the note at the top – add one to the number (after February) if it’s a leap year!
It’s available for download below: