Timer based on two dates (condition based)

I have a use case to trigger timer based on the condition.
I have two dateTime (camunda variables): common ETA (camunda variable: “registrationETA”) and ETA specific to the step (camunda variable: “googleRegistrationETA”).
“registrationETA” is mandatory whereas “googleRegistrationETA” is optional in case one wants to set a specific ETA for google use case.

Problem–
If “googleRegistrationETA” is set then I want to trigger timer on this variable, else I want to trigger timer on value set as common ETA “registrationETA”. The way I know to trigger timer is this: – R/${dateTime().parse(registrationETA).minusDays(10).toString()}/PT24H

You can use an expression to determine the entire timer definition. So if you create an expression to a bean, providing both variables as parameters, in that method implementation you can choose which gets applied and for each of them you can dynamically build up the actual timer definition.