Hi Jan,
SAP EM customizing allow you to display according to user choice. The custom table has to be called to retrieve the correct timezone to convert data and time.
Please check below discussion that help you to resolve.
Display planned/actual events in local (user) t... | SCN
Regards
GGOPII
P.S:
Go to below link on page 32 to check "Logic to sync two Timezone Comboboxes"