It stopped working and on start up the console gives the error below.
My public server is at a host which is using Linux. I ran this on a test server on a windows PC and it does work. I had exactly the same plugins and settings on the test server. Is there perhaps something wrong with the Linux version of Oxide?

(10:21:26) | Failed to run a 120.00 timer in 'PlaytimeSupplySignal v1.1.3' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.PlaytimeSupplySignal.CountDaTime (System.Collections.Generic.List`1[T] playerlist) [0x00048] in <595c972e2ead4d30b7322c73cae8023c>:0

  at Oxide.Plugins.PlaytimeSupplySignal.LetsClockOnActivePlaytime () [0x0000b] in <595c972e2ead4d30b7322c73cae8023c>:0

  at Oxide.Plugins.PlaytimeSupplySignal.<OnServerInitialized>m__0 () [0x00000] in <595c972e2ead4d30b7322c73cae8023c>:0

  at Oxide.Core.Libraries.Timer+TimerInstance.FireCallback () [0x00018] in <c2afd8354b8b4f3ca451cf5a1aa111c3>:0​