Working version

src/resolvers/datetime-type.ts

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+import { GraphQLScalarType } from 'graphql';
+import { Kind } from 'graphql/language';
+
+export const DateTimeScalar = new GraphQLScalarType({
+  name: 'DateTime',
+  description: 'Date custom scalar type',
+  parseValue(value) {
+    return new Date(value); // value from the client
+  },
+  serialize(value) {
+    return new Date(value).toISOString(); // value sent to the client
+  },
+  parseLiteral(ast) {
+    if (ast.kind === Kind.INT) {
+      return parseInt(ast.value, 10); // ast value is always in string format
+    }
+    return null;
+  },
+});

src/resolvers/index.ts

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+import orders from './order';
+import { DateTimeScalar } from './datetime-type';
+
+const resolvers = {
+  Query: { ...orders },
+  DateTime: DateTimeScalar,
+};
+
+export default resolvers;

src/resolvers/order.ts

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+// (parent, args, ctx, info)
+const orders = {
+  orders: (_, __, { dataSources }) => dataSources.orderApi.getOrders(),
+  order: (_, { id }, { dataSources }) => dataSources.orderApi.getOrder(id),
+};
+
+export default orders;