add time zone support for sites and users

Oqtane.Client/Services/Interfaces/ITimeZoneService.cs

@@ -0,0 +1,18 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Oqtane.Models;
+
+namespace Oqtane.Services
+{
+    /// <summary>
+    /// Service to store and retrieve <see cref="TimeZone"/> entries
+    /// </summary>
+    public interface ITimeZoneService
+    {
+        /// <summary>
+        /// Get the list of time zones
+        /// </summary>
+        /// <returns></returns>
+        Task<List<TimeZone>> GetTimeZonesAsync();
+    }
+}