Date Arithmetic

Calculate date differences and add/subtract time by working with millisecond timestamps.

Example

const ONE_DAY = 24 * 60 * 60 * 1000;

// Days between two dates
const start = new Date("2024-01-01");
const end   = new Date("2024-12-31");
const days = Math.round((end - start) / ONE_DAY); // 365

// Add days to a date
function addDays(date, n) {
  return new Date(date.getTime() + n * ONE_DAY);
}
addDays(new Date("2024-06-01"), 30); // July 1, 2024

// Subtract months
const d = new Date("2024-06-15");
d.setMonth(d.getMonth() - 1); // May 15, 2024

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Pro Tip

Avoid adding days with setDate() across month boundaries — it overflows correctly but is harder to read than timestamp arithmetic.

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