Fixed: Dates before 1970 causing an exception.

Fixes #1913

src/NzbDrone.Common/Disk/DiskProviderBase.cs

@@ -294,12 +294,22 @@ namespace NzbDrone.Common.Disk
         {
             Ensure.That(path, () => path).IsValidPath();
 
+            if (dateTime.Before(DateTimeExtensions.Epoch))
+            {
+                dateTime = DateTimeExtensions.Epoch;
+            }
+
             Directory.SetLastWriteTimeUtc(path, dateTime);
         }
 
         public void FileSetLastWriteTime(string path, DateTime dateTime)
         {
             Ensure.That(path, () => path).IsValidPath();
+            
+            if (dateTime.Before(DateTimeExtensions.Epoch))
+            {
+                dateTime = DateTimeExtensions.Epoch;
+            }
 
             File.SetLastWriteTime(path, dateTime);
         }

src/NzbDrone.Common/Extensions/DateTimeExtensions.cs

@@ -38,5 +38,7 @@ namespace NzbDrone.Common.Extensions
         {
             return dateTime >= afterDateTime && dateTime <= beforeDateTime;
         }
+        
+        public static DateTime Epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
     }
 }