begin implementing Tables API

Pilz.Networking.CloudProviders.Nextcloud/Extensions.cs

@@ -32,5 +32,15 @@ namespace Pilz.Networking.CloudProviders.Nextcloud
         {
             return (OcsApiUrlPath)string.Format(path, @params);
         }
+
+        public static DateTime UnixTimeMillisecondsToDateTime(this long? value)
+        {
+            return DateTimeOffset.FromUnixTimeMilliseconds(value ?? 0).DateTime;
+        }
+
+        public static long ToUnixTimeMilliseconds(this DateTime value)
+        {
+            return new DateTimeOffset(value).ToUnixTimeMilliseconds();
+        }
     }
 }