Command to change date to julian format

[lorel]

The date command has an option (date +%j) that will present today's date in julian format. I am almost sure there is a command in unix that will change an user inputed date into julian just like the date command does.
If there is such command, can anyone tell me what it is?
If there is no command I may have to write a C language script to so this, but there is not use in reinventing the wheel.

Thanks
Joe