feat(core): add date serial conversion utilities

lib/src/dates.dart

@@ -0,0 +1,86 @@
+final DateTime epochBase = DateTime.utc(1899, 12, 30);
+
+double serialFromDate(int year, int month, int day) {
+  final date = DateTime.utc(year, month, day);
+  return date.difference(epochBase).inDays.toDouble();
+}
+
+DateTime dateFromSerial(double serial) =>
+    epochBase.add(Duration(days: serial.floor()));
+
+double serialFromDateTime(DateTime date) {
+  final asUtcMidnight = DateTime.utc(date.year, date.month, date.day);
+  return asUtcMidnight.difference(epochBase).inDays.toDouble();
+}
+
+double todaySerial() => serialFromDateTime(DateTime.now());
+
+int daysInMonth(int year, int month) {
+  final firstOfNext = month == 12 ? DateTime.utc(year + 1, 1, 1) : DateTime.utc(year, month + 1, 1);
+  return firstOfNext.subtract(const Duration(days: 1)).day;
+}
+
+double edate(double serial, int monthOffset) {
+  final base = dateFromSerial(serial);
+  var year = base.year;
+  var month = base.month + monthOffset;
+  while (month > 12) {
+    month -= 12;
+    year++;
+  }
+  while (month < 1) {
+    month += 12;
+    year--;
+  }
+  final day = base.day < daysInMonth(year, month) ? base.day : daysInMonth(year, month);
+  return serialFromDate(year, month, day);
+}
+
+int datedif(double startSerial, double endSerial, String unit) {
+  final start = dateFromSerial(startSerial);
+  final end = dateFromSerial(endSerial);
+  switch (unit.toUpperCase()) {
+    case 'D':
+      return endSerial.floor() - startSerial.floor();
+    case 'M':
+      return _fullMonths(start, end);
+    case 'Y':
+      return _fullMonths(start, end) ~/ 12;
+    case 'MD':
+      final anchor = DateTime.utc(end.year, end.month, start.day);
+      final adjusted = anchor.isAfter(end)
+          ? DateTime.utc(end.year, end.month - 1, start.day)
+          : anchor;
+      return end.difference(adjusted).inDays;
+    case 'YM':
+      return _fullMonths(start, end) % 12;
+    case 'YD':
+      final anchor = DateTime.utc(end.year, start.month, start.day);
+      final adjusted = anchor.isAfter(end)
+          ? DateTime.utc(end.year - 1, start.month, start.day)
+          : anchor;
+      return end.difference(adjusted).inDays;
+    default:
+      throw ArgumentError('Unknown DATEDIF unit: $unit');
+  }
+}
+
+int _fullMonths(DateTime start, DateTime end) {
+  var months = (end.year - start.year) * 12 + (end.month - start.month);
+  if (end.day < start.day) months--;
+  return months;
+}
+
+int weekday(double serial, {int mode = 1}) {
+  final isoWeekday = dateFromSerial(serial).weekday;
+  switch (mode) {
+    case 1:
+      return isoWeekday % 7 + 1;
+    case 2:
+      return isoWeekday;
+    case 3:
+      return isoWeekday - 1;
+    default:
+      return isoWeekday % 7 + 1;
+  }
+}