Ah, now understand....Your 'poota is using JST, and the sim picks that up.
I suggest you use https://www.timeanddate.com
to set up a second clock using UTC, which is what all sim flightplans use, as far as I know. That will make it a lot easier to set your sim to the time period you want to check. UTC=GMT, but London time is not necessarily UTC - they use daylight saving.
I have no experience of X-Plane, but I did a lot of AI stuff in FS2004 and FSX - you can still see the bite marks in the ceiling