Free & Instant

Smart
Date Math

Measure gaps between dates, roll calendars forward or backward, or reveal any weekday—in three focused tabs.

Open calculator ↓
3 Tabs
Δ Day spans
± Calendar shifts
📆 Weekday finder

Pick a tool

📅

Choose a tab and tap the action button
to see results here

Result

How to Use This Calculator

  1. Open the tab that matches your question—difference, calendar shift, or weekday.
  2. Use the browser date pickers for consistent ISO calendar days.
  3. Press the action button under each tab to populate the summary panel.
📐 Formula

Formula & Methodology

View formula, sources, and explanation+
Day counts use noon UTC-stripped local dates to reduce DST edge noise. Month/year shifts rely on built-in calendar rolling (end-of-month clamping applies).

Weeks reported as floor(days ÷ 7); remaining days listed separately. Month differences count whole calendar month boundaries.

❓ FAQ

Date FAQ

  • Yes—JavaScript Date includes leap-day logic when rolling months or years.
  • Pinning around midday avoids some daylight-saving midnight surprises when differencing.
  • Full weeks are whole seven-day blocks; leftover days show in the details.
  • Not here—this tool counts calendar days only.
  • Nothing leaves your browser.