Since 1 January 2001 (Day 1 of 3rd Millennium) there were 23 years with 365 days, 6 of which were leap years with 366 days (i.e. 6 extra days). The leap years were calculated using following Javascript calendar-maker & recording the number of Leap Year Februarys with 29 days. Therefore, you must multiply 365 days by 23 years = 8395. Since 6 of those 23 years were leap years, you must add 6 additional days: 8395 + 6 = 8401.