I think it will be nice that Service Manager Change Management System supports showing warning message to the operator when Scheduled Implementation Start and Scheduled Implentation End timings are entered. The Change Management System while change is getting saved, validates that is there any other Change Request which has Affected CI overlap as well as Scheduled Implementation Window overlap.
When any clash is detected, a warning message is shown to operator for example, "There is another change request on the same Affected CI “XXXXX” with Change ID “Cxxxxx”, which is overlapping with your Change Implementation Window. Will you like to change your Scheduled Implementation Time or you will proceed? Please press “Reschedule” to change the times and “Proceed” to save the Change Request".