Uninstall plugin

Currently there is no automatic uninstall procedure.
As TIMESTAMP fields can be used as a replacement of DATETIME fields (regardless of the possible date range, see: The DATE, DATETIME, and TIMESTAMP Types for MySQL 5.6 ) , you may continue to use TIMESTAMP fields instead of DATETIME fields.

If you really need to revert TIMESTAMP fields to DATETIME fields you may use a backup of the field modifications: this backup is in the table 'glpi_plugin_timezones_dbbackups' and contains the complete SQL code needed to revert fields.
You may apply this code to revert your GLPI database to previous state.

But one more time: this revert is not mandatory: you may continue to use TIMESTAMP instead of DATETIME as long as the input dates fit in the TIMESTAMP range.

Sample of contents from this backup table:

date table_name alter_table
2015-07-02 01:02:40 glpi_alerts ALTER TABLE `glpi_alerts`
MODIFY `date` datetime NOT NULL
2015-07-02 01:02:40 glpi_authldaps ALTER TABLE `glpi_authldaps`
MODIFY `date_mod` datetime DEFAULT NULL
2015-07-02 01:02:40 glpi_authmails ALTER TABLE `glpi_authmails`
MODIFY `date_mod` datetime DEFAULT NULL
2015-07-02 01:02:40 glpi_budgets ALTER TABLE `glpi_budgets`
MODIFY `date_mod` datetime DEFAULT NULL
2015-07-02 01:02:40 glpi_calendars ALTER TABLE `glpi_calendars`
MODIFY `date_mod` datetime DEFAULT NULL
.. .. ..