The Gregorian calendar adds 29 February in years divisible by 4. Years divisible by 100 are not leap years, unless they are also divisible by 400. 2024 and 2028 are leap years. 2100 is not.
The date calculators on this site use that rule. The number of days between 1 January and 1 March is 59 in a common year and 60 in a leap year. Week numbers follow the ISO rule: weeks start on Monday, and week 1 is the week that contains the first Thursday of the year.