Future Date

This calculator adds a whole number of calendar days to a start date and walks the result forward one month at a time rather than doing pure arithmetic on a single day-count. Starting from the day you entered, it repeatedly checks how many days remain in the current month, using a lookup that also checks for a leap-year February (lines 4-9 and 18), consumes either the rest of the days you asked to add or the rest of the month, whichever is smaller, and if the month runs out first it rolls the day back to 1 and advances the month -- and the year, if the month passes December (lines 25-29). Because it walks month by month, the result is correct across every month-length boundary in the Gregorian calendar, including a leap-year February with 29 days instead of 28.

The number of Days to Add has by far the largest effect on the Result Day of the three date-component inputs, since it's the only variable actually driving the walk forward; Start Year, by contrast, moves the Result Year almost one-for-one, because with the calculator's practical range of days to add, the walk essentially never crosses more than a single year boundary, so the output year tracks the input year directly. Start Month and Start Day are floored and clamped to a specific range before the loop runs (lines 14-15); Start Year, by contrast, is only floored with a fallback default, not clamped to any range (line 13), so an implausibly large or negative year still reaches the walk unchecked. Start Day's clamp is to 1-31 regardless of which month is selected, so an invalid combination like day 31 in a 30-day month is accepted and effectively normalized forward into the next month by the same walk logic used for every other date.