Date field in view Filter Criteria, where date may be blank
The conditions I had set above were too many


content is published
AND content type is TYPE
AND Start date is blank OR Start date is not blank AND start date is &lt;= NOW
AND End date is blank OR End date is not blank AND end date >= NOW


and should have been:


content is published
AND content type is TYPE
AND Start date is blank OR start date is &lt;= NOW
AND End date is blank OR end date >= NOW


because the NOT blank is implied.

Using the 'And/Or, Rearrange' feature of the Filter Criteria, I was able to create 3 groups joined by AND's with Group 2 and 3 joined by OR's. Like so: