There is sometimes some confusion on how the system default timezone will interact with a users custom timezone and also how this default system timezone can be set on an instance.
It is also helpful to understand the hierarchy of the users and default timezone on an instance.
The default timezone for an instance is usually set upon the initial configuration of the instance. This value is set using the glide.sys.default.tz property in the instance. In a new instance this value is usually blank, so before an instance is released for public use this value should be set with a valid timezone value. The value set in this system property will be the value which is used to determine the display value for date/time values for most record types in the system. The setting for this value will not modify the actual value stored in the underlying field on the instance, but is simply used to modify the value for display purposes in a record. The value as stored in the field itself is stored in UTC format (Coordinated Universal Time).
If, for any reason, a default timezone is not set in this property on a ServiceNow instance, the system will then default to the time zone setting used by the Java Virtual Machine (JVM) as stored on the user's local machine.
If a user has a personalized time zone set in his ServiceNow user preferences (which is stored in that individual's User record on the instance), that value will override the setting as found in the instance's system property setting which is used for determination of the conversion to the date/time value which is displayed for that particular user for date/time values within records.
Changing the Default System Timezone Value
Occasionally, it may be necessary to change this default system timezone setting on the instance. Although this will not change the actual stored date/time values as stored in any records on the instance, it could cause confusion for any user who does not have a user timezone preference set, as suddenly these data/time values as displayed in records will show in a different timezone than previously. Thus, before making this change, particularly on a production instance, it is recommended to provide a global announcement to any and all users of the system that this will take place.
Users who have a timezone preference already selected will not notice any difference in the values displayed.
To change this value, the following steps can be performed:
Log into the instance with an account having admin rights to the instance.
Browse to the following location on the instance using the Menu Navigator: System Properties -> Basic Configuration.
In the Basic Configuration properties window, scroll to the "System timezone for all users unless overridden in the user's record" heading and select, from the pull down menu the appropriate timezone value you would like to set as the default for the instance (i.e. US/Pacific). If you notice the value currently set as showing "System", this indicates the value is actually currently being set from the local time one as found on your local machine.
After selecting the appropriate default timezone, click the Save button.
Note: Changing many of the system properties in the instance will cause an immediate cache clear which could potentially cause some performance issues, so this operation should be performed in a period of low traffic on the instance.