Please refer this
Based on above documentation while creating Creating or Altering Database its was set to UTC-7:00 (Mountain Standard Time US&Canada)
I have checked the below thread whcih is similar to me
If we change dbtimezone from -07:00 to +00:00 , is there any effect on old data? Please suggest
It should not affect any data.
For more information please check the below discussion.
It may help.