Solution 1: In rule engine make additional timeseries (alongside of normal value) that will be delta values of your cumulative flow. On dashboard use time window "Current day" and setup some widget with latest value to do SUM over all those deltas.

Solution 2: make custom widget that will do a bit of calculation in order to show data only from midnight until Date.now() regardless of chosen time window.

Btw. I am on PE and therefore sorry if I mentioned something that is not in CE.