eHow.com has an article on how to make the conversion (here). With those instructions it does not look like it would be too difficult using activities in Workflow to accomplish.

You will likely want to separate out the values of the date, I suggest using the token editor with either the formatting or regular expression options and creating tokens of the date and the month and the julian date.

Use the information at the beginning of the eHow link Blake gave you to figure out the leap years.

EDIT: This image I created only accounts for calculating the amount of days that have passed in the year. You will need to combine your information with the year in order to get the date you wanted.

Shelby,

The date format you are referring to is the 'Ordinal' date format (yyddd).  If you are looking for an easy way to tokenize this and many other date/time formats from a date/time value then please visit my website at www.qfiche.com.  On the Products page I have several custom workflow activities that I have developed.  One of the free ones is the 'Date Tokenizer'.  This custom workflow activity accepts any date/time value and generates tokens for just about any date/time format you can think of (including Ordinal Date and Julian Date.

The download link will provide a 30 day royalty free version of the Date Tokenizer custom workflow activity.  If the activity meets your needs then you can register and I will provide a key to unlock the full version.  (The activity is provided free in either case but registering it allows me to keep you updated on any new features.)

Let me know if you have any questions.

Nice and simple!  If I had read your original post correctly I would have noticed that you wanted to convert an Ordinal date back into a calendar date and not vice-versa!