[DrJJWMac]

Perhaps a different approach would help ... just as Due is set only when something has a definite due date based on forces external to you, Start should only be set when external forces limit when the task can physically be started (versus defining when you THINK that you might WANT to do the task). Then, during your daily (or weekly or ...) review, use flags to "activate" tasks that you need/want to do (today or whenever).

This will keep out those tasks that you have no desire to do as well as those that cannot be started even when you might want to. IOW, Start Date becomes a field to exclude untenable tasks (just as with Due Date) and flags become your "activator". This also allows you to activate tasks on a finer grain time scale -- for example, only that block that you want to do in the next hour.

HTH