Day and Date Calculations — Prelims Strategy

Prelims Strategy

Write the known date and target date first, and choose one counting direction. Split the interval into complete years, complete months and remaining days without overlapping segments. Before adding an extra day, check whether 29 February is inside that interval. For century years, apply the four-hundred rule rather than stopping at divisibility by four.

Reduce the elapsed total modulo seven. Add the shift for a later date and subtract it for an earlier date. Use a tiny sanity check: a date seven days later must have the same weekday, and a date one day earlier must move backward once. This catches inclusive-endpoint and direction errors quickly.

When two matching dates are in adjacent years, inspect the actual intervening February. March 2023 to March 2024 includes the leap day, while March 2024 to March 2025 does not. Do not apply a universal 28-year repetition shortcut across century exceptions. If the problem uses historical dates without specifying a civil adoption rule, make the Gregorian assumption explicit rather than inventing a skipped-date correction.