How now SLA works if you applying SLA not one time:
As example priority of a ticket raised
Or the Ticket was reevaluated
If a new SLA is applied later in a conversation and it is different from any previous SLA, the timer will start from the most recent user message, not from the beginning of the Conversation. For instance, if a user message at 20:30 triggers a new SLA, the timer for this SLA will start from that message at 20:30. It's important to note that if the same SLA is reapplied, the timer does not restart but continues from where it left off, adding the elapsed time from the previous SLA to the current one 😄
For me correct way would be to always count SLA from time when customer has contacted at least in Tickets
