are you using the bundled UIM CABI or the stand alone?
CABI is built on the jaspersoft engine....
I found the following link that might help
https://community.jaspersoft.com/questions/1096911/how-addenable-time-zone-jasper-reports-schedule
for the UIM bundled I was able to find this information in the following location.
<Nimsoft>\probes\service\wasp\webapps\cabijs\WEB-INF\applicationContext.xml
<bean id="userTimeZonesList" class="com.jaspersoft.jasperserver.war.common.JdkTimeZonesList">
<property name="timeZonesIds">
<list>
<value>America/Los_Angeles</value>
<value>America/Denver</value>
<value>America/Chicago</value>
<value>America/New_York</value>
<value>Europe/London</value>
<value>Europe/Berlin</value>
<value>Europe/Bucharest</value>
</list>
</property>
</bean>