Setting a default runbook timezone is easily done, and can be set by any user with the Runbook Admin role type at minimum.
Set a default timezone
Click the Runbook Details icon on the right-hand tools panel. Next, select your desired timezone from the Default runbook timezone dropdown menu.
Once a timezone has been set, all timings within a runbook will be shown in the selected default runbook timezone. The default runbook timezone will be visible on all screens.
If you are viewing the runbook from a different timezone, you will see the default timezone and your local timezone displayed side by side in the task list.
Even with a default timezone set, any user can choose to view a runbook in the timezone of their choice. This will only affect their own view of the runbook's timings.
To do this, click Set Timezone in the top tools panel and select the desired timezone from the dropdown menu.