diff --git a/src/NmeaParser/Nmea/Gga.cs b/src/NmeaParser/Nmea/Gga.cs
index 831737e..c98e456 100644
--- a/src/NmeaParser/Nmea/Gga.cs
+++ b/src/NmeaParser/Nmea/Gga.cs
@@ -51,7 +51,7 @@ namespace NmeaParser.Messages
             if (!double.IsNaN(timeInSeconds))
                 TimeSinceLastDgpsUpdate = TimeSpan.FromSeconds(timeInSeconds);
             else
-                TimeSinceLastDgpsUpdate = TimeSpan.MaxValue;
+                TimeSinceLastDgpsUpdate = null;
             if (message[13].Length > 0)
                 DgpsStationId = int.Parse(message[13], CultureInfo.InvariantCulture);
             else
@@ -116,7 +116,7 @@ namespace NmeaParser.Messages
         /// <summary>
         /// Time since last DGPS update (ie age of the differential GPS data)
         /// </summary>
-        public TimeSpan TimeSinceLastDgpsUpdate { get; }
+        public TimeSpan? TimeSinceLastDgpsUpdate { get; }
 
         /// <summary>
         /// Differential Reference Station ID
diff --git a/src/NmeaParser/Nmea/Gns.cs b/src/NmeaParser/Nmea/Gns.cs
index a2183eb..ed4c1d6 100644
--- a/src/NmeaParser/Nmea/Gns.cs
+++ b/src/NmeaParser/Nmea/Gns.cs
@@ -155,7 +155,7 @@ namespace NmeaParser.Messages
             if (!double.IsNaN(timeInSeconds))
                 TimeSinceLastDgpsUpdate = TimeSpan.FromSeconds(timeInSeconds);
             else
-                TimeSinceLastDgpsUpdate = TimeSpan.MaxValue;
+                TimeSinceLastDgpsUpdate = null;
             if (message[11].Length > 0)
                 DgpsStationId = message[11];
 
@@ -247,7 +247,7 @@ namespace NmeaParser.Messages
         /// <summary>
         ///  Age of differential data - <see cref="TimeSpan.MaxValue"/> if talker ID is GN, additional GNS messages follow with GP and/or GL Age of differential data
         /// </summary>
-        public TimeSpan TimeSinceLastDgpsUpdate { get; }
+        public TimeSpan? TimeSinceLastDgpsUpdate { get; }
 
         /// <summary>
         /// eference station ID1, range 0000-4095 - Null if talker ID is GN, additional GNS messages follow with GP and/or GL Reference station ID