Yes it can be achieved using Agents.
Below are pointers to achieve it:-
1)Create a connectSOAP/REST depending on the kind of integration you require to connect to external system.
2)Create an activity to use your connector
3)Use expression builder to calculate the dateDifference
4)Send the mail using one of numerous OOTB email related activity that we have.
5)Create two agent and schedule them for two different time with above created activity.