When creating fields or field groups, the timezone for the display in the date fields (created, modified) is already taken care of in the xmls by the use of filter="user_utc".
Therefore the code is useless in the models as it applies this timezone a second time.
Testing Instructions
Select a time zone and check the results displayed for a new field and a new field group (save twice to obtain the modified date)
before patch tmz applyed twice
after patch is correct