adding custom timer

ZitiUpdateService/utils/CustomTimer.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace ZitiUpdateService.Utils {
+    class CustomTimer : IDisposable {
+        private readonly TimerCallback _realCallback;
+        private readonly Timer _timer;
+        private TimeSpan _period;
+        private DateTime _next;
+
+        public CustomTimer(TimerCallback callback, object state, TimeSpan dueTime, TimeSpan period) {
+            _timer = new Timer(Callback, state, dueTime, period);
+            _realCallback = callback;
+            _period = period;
+            _next = DateTime.Now.Add(dueTime);
+        }
+
+        private void Callback(object state) {
+            _next = DateTime.Now.Add(_period);
+            _realCallback(state);
+        }
+
+        public TimeSpan Period => _period;
+        public DateTime Next => _next;
+        public TimeSpan DueTime => _next - DateTime.Now;
+        public bool Change(TimeSpan dueTime, TimeSpan period) {
+            _period = period;
+            _next = DateTime.Now.Add(dueTime);
+            return _timer.Change(dueTime, period);
+        }
+
+        public void Dispose() => _timer.Dispose();
+    }
+
+    class TimerState {
+        public CustomTimer _timer;
+        public Checkers.ZDEInstallerInfo zdeInstallerInfo;
+    }
+}