add custom DateTime scalar type

src/scalars/DateTime.ts

@@ -0,0 +1,28 @@
+import { GraphQLScalarType } from "graphql/type";
+import { Kind } from "graphql/language";
+
+// See Apollo documentation: https://www.apollographql.com/docs/apollo-server/schema/custom-scalars#providing-custom-scalars-to-apollo-server
+export const DateTime = new GraphQLScalarType({
+  name: 'DateTime',
+  description: 'DateTime custom scalar type',
+  serialize(value) {
+    if (value instanceof Date) {
+      return value.getTime();
+    } else if (value instanceof Number) {
+      return value;
+    }
+    throw Error('GraphQL Date Scalar serializer expected a `Date` object or number');
+  },
+  parseValue(value) {
+    if (typeof value === 'number') {
+      return new Date(value);
+    }
+    throw new Error('GraphQL Date Scalar parser expected a `number`');
+  },
+  parseLiteral(ast) {
+    if (ast.kind === Kind.INT) {
+      return new Date(parseInt(ast.value, 10));
+    }
+    return null;
+  },
+})