Determine the day and year you want to convert to a star date. You will use this date to determine the alternate reality date corresponding to the dates used in the 2009 Star Trek movie. The movie takes place in an alternate time line parallel to the original series' time line.
Determine the first four digits. In The Next Generation, the star date was five digits. For alternate reality, use the number of digits in the year. The first digits are the year you are referring to. For example, if the year is 2010, then the first four digits are 2010.
Determine the number that comes after the decimal point. In alternate reality, this number represents the day of the year you are referring to. For example, if the day is January 16, the number after the decimal point is 16. So far, you have the star date as being 2010.16.
If you are calculating a date later in the year, you would count the days in each month, adding them together until you have the number of days into the year you are calculating. For example, if you want the year 2010 and the day October 16, the star date would be 2010.289.