Date Calculator

Date calculator: fast differences and precise date math

1) Core idea

Two modes: (a) days between two dates with an optional “include end day”, and (b) add/subtract years, months, weeks, days from a base date.

2) Flow you can trust

• Calendar pickers enforce DD/MM/YYYY.
• Calendar difference is computed in years → months → days for real‑world month lengths.
• Totals use exact millisecond differences; “include end day” adds one day to the final count.
• Add/Subtract applies years, months, then weeks/days for realistic rollovers.

3) Sanity checks

• End date must not precede start date.
• Month lengths and leap years are handled automatically.
• Local timezone can affect boundaries near midnight.

4) Shortcuts

• Tick “Include end day” for inclusive ranges (e.g., project spans).
• Use weeks when planning sprints; switch to days for exact deadlines.

5) Pitfalls

• Adding months to the 31st lands on the last valid day in shorter months.
• Cross‑DST changes can shift local clock time though calendar math remains correct.

6) Micro examples

• 01/01/2024 → 01/03/2024 = 0 years, 2 months, 0 days; total days 60 (leap year).
• Add 1 month to 31/01/2025 = 28/02/2025 (last day of Feb in 2025).

7) Mini‑FAQ

• What’s “include end day”? Treats both start and end as counted days; adds +1 to the day total.
• Why do calendar units differ from total days? Months vary in length; totals are pure time delta.

8) Action tip

Save a base date; iterate add/subtract values to test alternative schedules without retyping.