How to Count Days and Weeks Correctly
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Elapsed days or dates counted?
One day elapses from January 1 to January 2. If you count both calendar dates, the total is two. Calculators can disagree because one measures the interval while another includes both endpoints.
Use the difference between dates to measure elapsed time. Include both endpoints when you need to count every date in a period. Rules for legal, business, or contractual deadlines can differ; an arithmetic date calculation does not determine those rules.
Calendar days and weekdays
Calendar days include every day. A Monday-to-Friday count excludes Saturdays and Sundays, but does not automatically exclude public holidays. It therefore cannot by itself represent the working-day schedule of every organization or location.
These calculations use calendar dates without a time of day. A date still counts as one day even when a daylight-saving change makes that local day 23 or 25 hours long.
A week number may belong to another year
Under ISO 8601, weeks start on Monday and week 1 contains the year's first Thursday. Near New Year's Day, the ISO week-year may differ from the calendar year.
January 1, 2021, for example, belongs to week 53 of ISO year 2020. Keep the ISO year alongside the week number when grouping data. “Week 1” alone does not identify a unique period.
Convert minutes into decimal hours
An hour has 60 minutes. Seven hours and 30 minutes equals 7 + 30/60, or 7.5 decimal hours. By contrast, 7.30 decimal hours equals seven hours and 18 minutes. Identify the format before adding or comparing durations.