Common Day of Week Mistakes

Mistake One: Skipping the Century Rule

Treating any year divisible by 4 as a leap year. 1900 and 2100 are not leap years. If the year ends in 00 and is not divisible by 400, February has 28 days and any weekday calculation needs to reflect that.

Mistake Two: Forgetting the January Reduction

In Sakamoto's algorithm a January or February date must have its year reduced by one before calculating. Omitting that step shifts the answer by exactly one day and produces a confident but wrong weekday.

Mistake Three: Assuming 29 February Always Exists

Entering 29 February in a non leap year and letting it roll into March. The date does not exist, and a silent rollover hides the mistake rather than surfacing it.

Mistake Four: Mixing Week Conventions

Comparing an ISO week number from a European system against an American week number and concluding the data is wrong. Check the convention before assuming there is an error.

Mistake Five: Transposing Day and Month

Entering 3 April as 4 March. Both are valid dates producing different weekdays, and the calculator cannot detect a transposition, so checking the formatted result against the intended date is the safeguard.

Mistake Six: Assuming Every Year Has 52 Weeks

Some ISO years contain 53 weeks, which is why a fiscal year of 52 weekly periods does not always span exactly 364 days of dates.