Date Difference Calculator

Calendar-style breakdown of the span between two dates:

1. Sort the inputs so the earlier date comes first; the order you typed them in does not matter.
2. Tentatively take the year, month, and day differences: $\text{years} = \text{laterYear} - \text{earlierYear}$, $\text{months} = \text{laterMonth} - \text{earlierMonth}$, $\text{days} = \text{laterDay} - \text{earlierDay}$.
3. If the day count is negative, the later date has not yet reached the same day-of-month as the earlier date — borrow the days of the previous month and subtract one from the months count.
4. If the months count is then negative, the later month is still before the earlier month within the year — borrow 12 months and subtract one from the years count.
5. Compute total elapsed time directly from the timestamps: $\text{totalDays} = \Delta t / 86400000$, $\text{totalWeeks} = \text{totalDays} / 7$, $\text{totalHours} = \text{totalDays} \times 24$. Dates are parsed as UTC midnight to avoid daylight-saving off-by-ones across the span.