I figured out what was going on. I didn't need to create a new instance of EquipmentTask (Duh):
var t = iEquipmentTaskRepository.GetSingle(maintenanceViewModel.TaskId);
EquipmentTask equipmentTask = new EquipmentTask(); // <-- this line was not necessary
equipmentTask.TaskId = maintenanceViewModel.TaskId;
equipmentTask.MaintenanceDueDate = nextDueDate;
I wasn't adding a new instance, just updating an existing one. Once I removed the line, it worked:
var equipmentTask = iEquipmentTaskRepository.GetSingle(maintenanceViewModel.TaskId);
equipmentTask.TaskId = maintenanceViewModel.TaskId;
equipmentTask.MaintenanceDueDate = maintenanceViewModel.NextDueDate;