Ah. I learn something new every day on PM.
George
May as well shit in mad-Phil's personal bowl of oatmeal twice in a row, then. He 'gets off' on that.
The 'conventionally utilized' Julian date is but the sequential day of a given year.
Pedantically, it is the wrong term, but it is what is widely used - unless, perhaps, one is an Astronomer.
There are sources all over town from the paper calendars and day-timers to a small number on the corner of the display of some of those 'atomic' clocks one can place on a desk or hang on the wall.
The 'real' Julian date is a continuously incrementing and far larger number.
US Navy Observatory provides it online, along with calculators for ascertaining what it was or will be, given a year, month, day, hour minute, and second of prior or future common calendar years - or the reverse:
Julian Date Converter
An instructional overview with a precis of the history is also provided on that page as text. Links to API's too.
If one can trust a given machine's local 'system clock', especially if the system clock is being kept updated over a link to a 'good enough' tier of time-servers, either/both Julian date or day-of-year may be extracted with a built-in function.
The OP is by no means the first Pilgrim to ever need such a service.
If he is already programming the rest of his project, he can do this part without injection of kinky-f**kery abuse from 'downunder' as well.
Side note: I have NO idea if mad-Phil is actually stroking himself as he posts or not. Fortunately, Australia is sufficiently remote from PM's server that detail does not HAVE TO pass the proverbial 'sniff test'.