Add date difference calculator

date-diff.ts

@@ -0,0 +1,53 @@
+// https://stackoverflow.com/a/49201872
+const calcDateDifference = (
+  start: string | Date,
+  end: string | Date
+): { year: number; month: number; day: number } => {
+  // Convert to true dates without time
+  let startDate = new Date(new Date(start).toISOString().substring(0, 10));
+  let endDate = new Date(
+    (end ? new Date(end) : new Date()).toISOString().substring(0, 10)
+  );
+
+  // If start date comes after end, flip them around
+  if (startDate > endDate) {
+    const swap = startDate;
+    startDate = endDate;
+    endDate = swap;
+  }
+
+  // Account for leap years
+  const startYear = startDate.getFullYear();
+  const february =
+    (startYear % 4 === 0 && startYear % 100 !== 0) || startYear % 400 === 0
+      ? 29
+      : 28;
+  const daysInMonth = [31, february, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
+
+  // Create year and month diffs
+  let yearDiff = endDate.getFullYear() - startYear;
+  let monthDiff = endDate.getMonth() - startDate.getMonth();
+  if (monthDiff < 0) {
+    yearDiff--;
+    monthDiff += 12;
+  }
+
+  // Create day diff while adjusting others
+  let dayDiff = endDate.getDate() - startDate.getDate();
+  if (dayDiff < 0) {
+    if (monthDiff > 0) {
+      monthDiff--;
+    } else {
+      yearDiff--;
+      monthDiff = 11;
+    }
+    dayDiff += daysInMonth[startDate.getMonth()];
+  }
+
+  // Return true differentials
+  return {
+    year: yearDiff,
+    month: monthDiff,
+    day: dayDiff,
+  };
+}