Codes

get day of year from year,month,day,...

datetime.datetime(year,month,day,0,0,0).strftime('%j')

day of year to date

d=datetime.date(int(start_year), 1, 1) + datetime.timedelta(int(dataset[1][9:12]) - 1)
start_year = d.year
start_month = d.month
start_day = d.day

datetime to timestamp

time.mktime(datetime.datetime(start_year, start_month, start_day, 0, 0, 0).timetuple())