Kefiw

Archived noindex page. Kefiw's public focus is Property decision help.

Archived page

This older Kefiw page is kept for reference, marked noindex, and removed from the primary sitemap. The current Kefiw experience is focused on property decisions: cost, quotes, damage, buying, selling, owning, and packets.

Go to Property

Date Difference Errors: Leap Days, Off-by-One, DST

Five errors that break invoices, contracts, and legal deadlines.

Avoid the five common errors and your date math stops costing interest, fees, and contract fights.

Contracts and invoices hinge on exact day counts. The common errors are predictable: inclusive-vs-exclusive off-by-one, missed leap days, month-end arithmetic, and DST-shift boundary effects.

Quick answer

Avoid the five common errors and your date math stops costing interest, fees, and contract fights.

What you are trying to do
Five errors that break invoices, contracts, and legal deadlines.
Best next step
Date Difference Calculator
Limit to remember
Treat this as a practical aid for the task, not a replacement for professional judgment.

Key points

  • ▸ Inclusive vs exclusive: from Jan 1 to Jan 10 is 9 days (exclusive) or 10 days (inclusive). Specify which convention.
  • ▸ Leap-day miss: 2020 and 2024 add Feb 29. Span 2019-01-01 to 2025-01-01 = 2,193 days, not 2,190 (3 leap days).
  • ▸ Month-end quirk: Jan 31 + 1 month = Feb 28/29. Different libraries handle end-of-month differently.
  • ▸ DST: "24 hours" can be 23 or 25 hours on DST transition days. Day-counts usually unaffected, but hour-level math breaks.
  • ▸ 29 Feb to 29 Feb: spans non-leap years need a convention. Usually 4 years, exactly.

Examples

  • Invoice dispute
    "Net 30" from 2024-02-14 is 2024-03-15 (exclusive) or 2024-03-16 (inclusive)? Misunderstanding costs a day's interest.
  • Leap day error
    Mental math: "2020-01-01 to 2024-01-01 is 4 × 365 = 1,460 days." Wrong — it is 1,461 (one leap day, 2020).
  • Month-end bug
    Subscription starts Jan 31, monthly. Next charge: Feb 28? Mar 1? Different rules in different systems.

When to use which tool

Related

Frequently asked questions

› Is "30 days" the same as "1 month"? Trust & accuracy

No. "30 days" is exact; "1 month" is 28-31 depending on month. Contracts should say one or the other, not both.

› How do I count business days? How-to

Standard date-diff gives calendar days. Business-day count (excluding weekends and holidays) needs a separate tool.

› How should I use this guide with a Kefiw tool? How-to

Use the guide as the plan and the linked Kefiw tool as the check. Read the steps first, try the move manually, then use the tool to compare outputs, catch edge cases, and decide whether the result actually fits your task.

› What mistake do tool guides help avoid? Troubleshooting

Tool guides help avoid using a utility mechanically without understanding what you are trying to accomplish. Most word, writing, and text utilities are fast, but speed can hide context mistakes. Know whether you are solving a puzzle, cleaning copy, drafting a line, or checking a rule.

› Can a tool guide help me learn the skill? How-to

A tool guide can help you learn if you pause before accepting the output and ask why it worked. Compare your first guess with the tool result, look for the rule or pattern, and repeat that review. Passive copying solves one task; active review builds the skill.