A leap day changes a duration without moving a calendar anniversary. That distinction matters when a couple wants both a yearly celebration and a fixed number of completed days.
Start on March 1, 2023 and calculate until February 29, 2024. The result is 365 elapsed days and inclusive Day 366. The first calendar anniversary is March 1, 2024, one day later.
Use the appropriate target
Choose elapsed counting if “365 days together” means 365 complete date intervals. Save February 29 as a one-time milestone. Save March 1 as an annual relationship anniversary.
If the phrase means inclusive Day 365, the target is February 28, 2024. Counting the start as Day 1 moves each numbered-day target one day earlier. This adjustment is independent of leap years.
Explain the calendar-year output
On February 29, the calendar-year count is still zero because March 1 has not arrived. A full year is reached on the anniversary even though the elapsed day total is already 365.
Avoid treating “a year” as an interchangeable label for any 365-day period. When sharing a countdown, include the target date so a partner can check which convention was used before arranging a celebration.