Is it possible to set the default Date & time format for all users in the system? Or at least change the format connected with the language? (it is not possible to set format for the user you can select the language with the default format) 
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Hello Paulina,
Thank you for your question!
Unfortunately, there is no basic system setting to change the time zone for already existing system users.
You can use the SQL script for this purpose.
Please note that the DateTimeFormatId column value refers to the SysLanguage table and the TimezoneId value refers to the Code column in Timezone table so the SQL script will be the following:
UPDATE SysAdminUnit SET DateTimeFormatId = 'value from SysLanguage', TimeZoneId = 'Code column value from TimeZone table' WHERE ContactID IS NOT NULL
All you need to do is to add the proper IDs you'd like to change the values to.
For example, here is the script with values which will set Date and Time format for all users to English and timezone to GMT:
UPDATE SysAdminUnit SET DateTimeFormatId = '910eb38b-c00f-4d84-8e4a-853a62476b68', TimeZoneId = 'GMT Standard Time' WHERE ContactID IS NOT NULL
Regards,
Orkhan