test(functions): add date and financial tests

test/functions_date_test.dart

@@ -0,0 +1,49 @@
+import 'package:cellar/cellar.dart';
+import 'package:test/test.dart';
+
+CellValue ev(String formula) {
+  final sheet = Sheet();
+  sheet.setValue('Z1', formula);
+  return sheet.valueOf('Z1');
+}
+
+double num(CellValue value) => (value as NumberValue).value;
+
+void main() {
+  group('date functions', () {
+    test('DATE produces the Excel serial number', () {
+      expect(num(ev('=DATE(2024, 1, 1)')), 45292);
+      expect(num(ev('=DATE(1899, 12, 31)')), 1);
+    });
+
+    test('YEAR MONTH DAY decompose a serial', () {
+      expect(num(ev('=YEAR(DATE(2024, 3, 15))')), 2024);
+      expect(num(ev('=MONTH(DATE(2024, 3, 15))')), 3);
+      expect(num(ev('=DAY(DATE(2024, 3, 15))')), 15);
+    });
+
+    test('DATEDIF across units', () {
+      expect(num(ev('=DATEDIF(DATE(2020,1,1), DATE(2024,3,15), "Y")')), 4);
+      expect(num(ev('=DATEDIF(DATE(2020,1,1), DATE(2024,3,15), "M")')), 50);
+      expect(num(ev('=DATEDIF(DATE(2020,1,1), DATE(2024,3,15), "YM")')), 2);
+      expect(num(ev('=DATEDIF(DATE(2024,1,1), DATE(2024,1,11), "D")')), 10);
+    });
+
+    test('EDATE and EOMONTH honour month lengths', () {
+      expect(num(ev('=DAY(EDATE(DATE(2024,1,31), 1))')), 29);
+      expect(num(ev('=DAY(EOMONTH(DATE(2024,2,10), 0))')), 29);
+      expect(num(ev('=MONTH(EOMONTH(DATE(2024,1,15), 2))')), 3);
+    });
+
+    test('WEEKDAY and DAYS', () {
+      expect(num(ev('=WEEKDAY(DATE(2024,1,7))')), 1);
+      expect(num(ev('=WEEKDAY(DATE(2024,1,8), 2)')), 1);
+      expect(num(ev('=DAYS(DATE(2024,1,10), DATE(2024,1,1))')), 9);
+    });
+
+    test('TODAY agrees with the system year', () {
+      final year = DateTime.now().year;
+      expect(num(ev('=YEAR(TODAY())')), year.toDouble());
+    });
+  });
+}