See our new duration custom field document in which we describe how to add that custom field.
As of version 9.0.0, Time String Custom Field is named "Duration Field".
Defining "TTS - Time String Custom Field"
Click on administration cog icon and then select Issues
Click Custom Fields from the left side menu.
Click Add Custom Field button.
Select either All or Advanced to see the TTS - Time String Custom Field.
Find and select TTS - Time String Custom Field from the menu and click Next
Type the name and description for the field.
Select the screens for the newly added custom field and click Update
All done, now this custom field will be visible in the SLA Configuration screen while adding or updating an SLA.
When this field is used for multiple SLAs with different calendars in an issue, every SLA will evaluate this field according to its own calendar. If you would like to obtain same durations for all SLAs make sure that field value does not includes days since length of a business day might differ for different calendars.