Formula and method
Elapsed calendar days are computed from UTC-normalized date values to avoid daylight-saving clock changes.
Worked example
Adding 30 days to 1 January 2026 gives 31 January 2026.
Assumptions
- Day difference excludes the start date unless inclusive mode is selected.
- Month addition clamps to the final valid day when necessary.
Reference sources
Reviewed and updated 2026-07-14