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Period Problems Calculator

Online weekday calculator. Pick today and enter the elapsed days to find the weekday after them, with a modulo-7 explanation—ideal for elementary date puzzles.

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Weekday=(today+days) mod 7
Today
Days later

📖 Tutorial | Cycle

1. Definition
Events repeat with a period; use the remainder to locate position.
2. Formula & Notation
Period7 for weekdays
RemainderCount on
3. Properties & Laws
  • Weekday=(today+days) mod 7;
  • Remainder 0 same as start;
  • Uses elapsed days;
  • Across months and years.
4. Steps
  1. Pick today s, enter n;
  2. Click "Find";
  3. Or click "Load Example".
5. Worked Examples
Example: Wednesday in 10 days.
Solution: Saturday.
6. Common Pitfalls
Divide by 7;
0 unchanged;
Days non-negative.

❓ FAQ | Cycle

Cycle problem?
Remainder locates.
Wed +10?
Saturday.
Why 7?
Weekly period.
Remainder 0?
Same.
Across months?
Yes.
Remainder?
Count days.