My Used Case is to Send Email in 4 different dates in year like 1/10, 4/17, 5/20

So you are trying to dynamically change the timer's start event delay for when the process runs? I don't think you can do that. When the process model is published (and first run?) the values used in the constant/expression are set for the start timer and can't be changed. Alternatively, you could have the process run every day, and have it execute and immediately go to a XOR where you check a condition that the current date = your desired date. If it is the same, continue the process, else go to a terminate end event. Sure, the process will run daily now, but it effectively won't do anything. If you set the condition to check that the current date is in a string of dates that you store in a constant you could change the constant whenever you want and the next day the process will check for the current list of dates.

So you are trying to dynamically change the timer's start event delay for when the process runs? I don't think you can do that. When the process model is published (and first run?) the values used in the constant/expression are set for the start timer and can't be changed. Alternatively, you could have the process run every day, and have it execute and immediately go to a XOR where you check a condition that the current date = your desired date. If it is the same, continue the process, else go to a terminate end event. Sure, the process will run daily now, but it effectively won't do anything. If you set the condition to check that the current date is in a string of dates that you store in a constant you could change the constant whenever you want and the next day the process will check for the current list of dates.