Returns the calendar day of year, not the number of bars.
For example, a February 1 bar will always return 32. A December 31 bar will return 365 on normal years or 366 on leap years.
Negative offsets, e.g. DayOfYear[-10], can be legitimately used to obtain the day of year of a future bar. This works even if the offset goes beyond the range of the currently loaded data file. For best results when future dates are required, a HolidayList should also be provided.
