365 Resource mailbox wrong timezone for working hours

A newly created room (resource) mailbox in M365 then appeared for a user with the working hours set to the wrong time zone, i.e. the user’s regular calendars showed 8-5 as normal, but this new one showed working hours as 1600-0100. There isn’t – currently – a method of changing this within the admin center (that I’ve found anyway!), so PowerShell to the rescue once again .Connect to Exchange Online (PS command: Connect-ExchangeOnline):

# Add all available resource mailboxes into a variable
$rooms = Get-Mailbox -RecipientTypeDetails RoomMailbox

# Set mailbox Timezone, Language and Date & Time formats to UK
$rooms | ForEach-Object {Set-MailboxRegionalConfiguration $_.alias -TimeZone ‘GMT Standard Time’ -language ‘en-GB’ -DateFormat ‘dd/MM/yyyy’ -TimeFormat ‘HH:mm’}

# Update the calendar’s working hours start & end time and timezone to our UK corp standard
$rooms | ForEach-Object {Set-MailboxCalendarConfiguration $_.alias -WorkingHoursTimeZone ‘GMT Standard Time’ -WorkingHoursStartTime ’08:00:00′ -WorkingHoursEndTime ’17:00:00′}