Plain-language references
Date and time guides
Short explanations for the rules behind the calculators, with concrete dates and links back to the relevant tools.
Leap year rules in the Gregorian calendar
Understand why some years have February 29 and why 1900, 2000, and 2100 behave differently.
How ISO week numbers work
Learn the Monday-first ISO week system and why late December can belong to week 1 of the next year.
Should the end date be included?
See how inclusive and exclusive endpoints change day counts, weeks, weekdays, and calendar intervals.
Avoiding ambiguous date formats
Understand why 03/04/2025 can name two dates and how ISO 8601 reduces confusion.
Unix timestamps, UTC, and local time
Understand epoch seconds, milliseconds, UTC, and why local displays can differ for the same instant.