The MySQL STR_TO_DATE() function allows you to build a date value from the various date parts.
When using this function, you provide a string of the various date parts, and a second argument that specifies the format that the date is provided in.
When using MySQL, you can use the MAKEDATE() function to return a date from the year and day-of-year parts.
In other words, you provide two arguments; one being the year, and the other being the day-of-year. The MAKEDATE() function will then return the date value based on those two arguments.