What it does:
Atle's date to yearday convertor let's you type a date, and find out which yearday it is. You may also specify a year, to do a leapyear check to find out if 29th February should be counted with. If no year is defined, year 1 is used (non-leapyear).
You can also do a reverse calculation, type a yearday (and a year) to find the date it corresponds to.
To enable reverse mode (yearday->date).
To disable reverse mode (date->yearday).
Gives 61, since 2004 is a leapyear.
Enables yearday->date mode.
Gives 02 / 03 - 1, since no year is defined and a non-leapyear (1) is used.