Hi @Arno Cools
Welcome to the Microsoft Q&A platform!
You can make changes to the wrong time zone user in the following ways:
1.Connect to Exchange Online using PowerShell:
$UserCredential = Get-Credential
Connect-ExchangeOnline -UserPrincipalName $UserCredential.UserName -ShowProgress $true
2.Change the user's time zone:
$users = Get-Mailbox -ResultSize unlimited -Filter{(RecipientTypeDetails -eq <RoomMailbox>)}
$users | %{Set-MailboxRegionalConfiguration $_.Identity -TimeZone <Pacific Standard Time>}
For more details on the parameters, you can refer to:https://learn.microsoft.com/en-us/exchange/troubleshoot/user-and-shared-mailboxes/incorrect-missing-time-zone-settings
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