in_time_zone is a rails thing, not present in basic Ruby. But you do not need a ruby filter to do this, you can do it using a date filter. If you want to move from UTC to Europe/Brussels, which is Etc/GMT-1, then tell the date filter it is actually Etc/GMT+1, or Atlantic/Azores.
That's why you have to tell it the timezone of the log file has the opposite sign to the adjustment you want to apply. Atlantic/Azores, not Europe/Brussels.
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