Date arithmetic computes dates by adding or subtracting days and finding the interval between dates. Gregorian-calendar rules make even one-day movement nontrivial: month lengths, leap years, the missing year zero, and the 1582 calendar transition all matter. section 5.3
Two implementation strategies expose the trade-off between straightforward code and efficient arithmetic:
- Day-by-day iteration uses next_date() and previous_date() inside add_days() and days_from(). It is functional but inefficient because loops, calendar-rule calculations, comparisons, and temporary Date objects grow with the number of days. section 5.3.1
- Julian day numbers represent each day with one integer. Adding n becomes julian + n, and the interval becomes subtraction; conversion remains necessary when exposing year, month, and day. section 5.3.2
Counting between two dates
For September 2, 2025 and April 3, 2027, days_from() produces 578; adding 578 days to the first date returns April 3, 2027. The loop version reaches this result correctly, while the Julian-number version performs the arithmetic directly.
