feat(functions): add date and time functions

lib/src/functions/date.dart

@@ -0,0 +1,75 @@
+import '../dates.dart';
+import '../evaluator.dart';
+import '../values.dart';
+import 'helpers.dart';
+
+void registerDate(Map<String, CellFunction> registry) {
+  registry['DATE'] = (args, ctx) {
+    arity(args, 3, 3);
+    return NumberValue(serialFromDate(intArg(args[0], ctx), intArg(args[1], ctx), intArg(args[2], ctx)));
+  };
+
+  registry['TODAY'] = (args, ctx) {
+    arity(args, 0, 0);
+    return NumberValue(todaySerial());
+  };
+
+  registry['NOW'] = (args, ctx) {
+    arity(args, 0, 0);
+    final now = DateTime.now();
+    final dayFraction =
+        (now.hour * 3600 + now.minute * 60 + now.second) / 86400.0;
+    return NumberValue(todaySerial() + dayFraction);
+  };
+
+  registry['YEAR'] = (args, ctx) {
+    arity(args, 1, 1);
+    return NumberValue(dateFromSerial(numberArg(args[0], ctx)).year.toDouble());
+  };
+
+  registry['MONTH'] = (args, ctx) {
+    arity(args, 1, 1);
+    return NumberValue(dateFromSerial(numberArg(args[0], ctx)).month.toDouble());
+  };
+
+  registry['DAY'] = (args, ctx) {
+    arity(args, 1, 1);
+    return NumberValue(dateFromSerial(numberArg(args[0], ctx)).day.toDouble());
+  };
+
+  registry['WEEKDAY'] = (args, ctx) {
+    arity(args, 1, 2);
+    final mode = args.length > 1 ? intArg(args[1], ctx) : 1;
+    return NumberValue(weekday(numberArg(args[0], ctx), mode: mode).toDouble());
+  };
+
+  registry['DATEDIF'] = (args, ctx) {
+    arity(args, 3, 3);
+    final start = numberArg(args[0], ctx);
+    final end = numberArg(args[1], ctx);
+    final unit = textArg(args[2], ctx);
+    if (end < start) throw const EvalException(ErrorValue(ErrorKind.numError));
+    try {
+      return NumberValue(datedif(start, end, unit).toDouble());
+    } on ArgumentError {
+      throw const EvalException(ErrorValue(ErrorKind.value));
+    }
+  };
+
+  registry['EDATE'] = (args, ctx) {
+    arity(args, 2, 2);
+    return NumberValue(edate(numberArg(args[0], ctx), intArg(args[1], ctx)));
+  };
+
+  registry['EOMONTH'] = (args, ctx) {
+    arity(args, 2, 2);
+    final shifted = edate(numberArg(args[0], ctx), intArg(args[1], ctx));
+    final date = dateFromSerial(shifted);
+    return NumberValue(serialFromDate(date.year, date.month, daysInMonth(date.year, date.month)));
+  };
+
+  registry['DAYS'] = (args, ctx) {
+    arity(args, 2, 2);
+    return NumberValue((numberArg(args[0], ctx) - numberArg(args[1], ctx)).floorToDouble());
+  };
+}