The starting date
The browser reads the visitor's current local calendar date and normalizes it to local midnight. On a days-from-today page, today is day zero and tomorrow is day one. A custom start date replaces today when the visitor chooses one.
Calendar days and daylight saving time
Calendar-day calculations advance the date itself. They do not multiply the number of days by 24 hours. This prevents a daylight saving time change from shifting the answer to the previous or following calendar date. Weekends and holidays remain included unless a tool explicitly says it calculates business days.
Months, years, and month-end dates
Months and years are shifted using calendar fields. If the original day does not exist in the target month, the result uses that month's final valid day. For example, January 31 plus one month becomes February 28, or February 29 in a leap year.
Business days and holidays
The business-day comparison on current date pages skips Saturdays and Sundays. It does not remove national, regional, religious, or company holidays, because those differ by location and employer. The page labels this limitation beside the result.
Countdowns
A date countdown measures the remaining time until local midnight at the beginning of the target date. The date answer and countdown can therefore describe the same target in different ways: one as a calendar date and the other as remaining elapsed time.