Why finding the exact midpoint between two dates is trickier than it sounds

A developer set out to build a simple date calculator to find the midpoint between two people's birthdays, expecting it to be a quick arithmetic problem. Using the Luxon library for calendar-aware date math, the tool avoids common pitfalls like DST errors that arise from raw millisecond calculations. A key design challenge emerged around fencepost ambiguity — whether to count the start date inclusively or exclusively — which the tool resolves by spelling out its counting convention explicitly in the UI. The deeper problem is that when two dates are an odd number of days apart, no single calendar day sits exactly in the middle, so the tool returns a two-day range instead of one date. The project highlights how seemingly simple date calculations can hide genuine edge cases that require deliberate design decisions rather than straightforward division.