Calculate SLA times based on custom Start/End fields

Dear all,

I have 2 custom fields in my incident form: "Start Time" and "End Time", those fields are filled by operators (only for a specific category of incidents...)

I'd like to calculate SLA times based on these 2 fields. ServiceNow gives the possibility to set a retroactrive start time setting it to a specific field ("Set start to" field in SLA definition form).

I was wondering if there is the possibility to select also a datetime field to set the official "stop" of the SLA. Any info about? or any workaround to achieve the goal?

Thanks to all

Cheers

Ernesto