Unfortunately, functions are not available for trigger actions. You could create a Power Automate flow that starts on item (ticket) modification, checks conditions and then set custom value in the
Resolution date. Please note that HelpDesk sets the field automatically on closing a ticket. If you set a custom date before HelpDesk updated the field, it will be overwritten. So you will need either to set a delay in your flow to ensure that the field is not empty or to use a custom column to track resolution dates.
If you need help with configuring such a flow, please create a new topic. But anyway, I will ask developers whether it is possible to support functions in the trigger actions too.