refactored code to encapsulate date formatting

lib/services/api.dart

@@ -1,11 +1,13 @@
 import 'package:http/http.dart' as http;
 import 'dart:convert';
-import 'package:timezone/timezone.dart' as tz;
 
 import '../model/flood_station.dart';
 import '../model/reading.dart';
+import 'date_utility.dart';
 
 class Api {
+  Api._();
+
   static const String _rootUrl =
       'https://environment.data.gov.uk/flood-monitoring';
 
@@ -56,7 +58,8 @@ class Api {
   static Future<List<Reading>> fetchReadingsFromStation(
       String stationId) async {
     List<Reading> readings = [];
-    final dateTime = _getCurrentUKTime().subtract(Duration(days: 1)).toUtc();
+    final dateTime =
+        DateUtility.currentUKTimeUtc.subtract(Duration(days: 1)).toUtc();
     final url =
         '$_rootUrl/id/stations/$stationId/readings?since=${dateTime.toIso8601String()}&_sorted';
     final response = await http.get(Uri.parse(url));
@@ -68,9 +71,4 @@ class Api {
     }
     return readings.reversed.toList();
   }
-
-  static DateTime _getCurrentUKTime() {
-    final london = tz.getLocation('Europe/London');
-    return tz.TZDateTime.now(london);
-  }
 }