Date and time guides

Plain-English explainers for the conventions behind the DaySums calculators: why day counts differ by one, why week numbers disagree, and what "5 business days" actually means.

Inclusive vs Exclusive Day Counting, Explained

Why "how many days between" has two right answers, and how off-by-one errors creep into deadlines, medication courses, and billing.

ISO Week Numbers: Why Your Week 1 Isn't My Week 1

How ISO 8601 defines week 1, why 1 January can belong to last year's week 53, and why US planners are often one week off.

Business Day Counting: The Conventions That Move Deadlines

Start-day exclusion, end-day inclusion, differing weekends, and holidays: the choices that make "5 business days" ambiguous.