Understanding ticket SLAs in detail

[SB87]

I'm fairly familiar with how SLAs work in ServiceNow. In summary, SLA definitions are created for a particular table e.g. Incident, Problem, etc. and the calculated priority of the ticket determines which SLA gets assigned.

However, what I am not entirely clear on the inner workings in how SLAs work. 

Scenario 1

• SLA Definition for Incident P1 created where SLA must be resolved in 4 hours (all stored in [contract_sla] table)
  • Duration = 4 Hours (user specified duration)
  • Target = Resolution
  • SLA has Schedule Source = SLA Definition
  • Schedule assigned = 08:00-18:00 Mon-Frid
  • Timezone source = caller timezone
  • Usual conditions set e.g. Active = True, Priority = 1)

If a P1 incident is then created, then I believe a record is created for the attached sla associated to the ticket in the [task_sla] table. However, what I am unclear about it how schedules come into effect. 

If the ServiceNow instance is located in a location using UTC and say the time at when the Incident is created is 17:00, then am I correct the SLA will pause based on the scheduled assigned at 18:00?

What is the callers timezone is 4 hours behind i.e. from their perspective the Incident was raised 13:00 their local time? Would the ticket SLA still pause at 18:00 UTC based on the assigned schedule set against the definition?

Is my understanding correct and is there any other key things I should be considering?

Thanks in advace,

Sal