This function assigns fake date information, beginning 01/01/startYear, to
each empirical timestamp. Users can control what format the output vector
is in by changing the dateFormat argument
(format: "mdy" = month-day-year, "ymd" = year-month-day,
"dmy" = day-month-year, or "ydm" = year-day-month).
This is a sub-function that can be found within datetime.append.