StartDate & DueDate are DateTime fields.
As the time used with the API is on UTC time zone, the date part alone may be incorrect (for example, you'll get Sep 20 23:00 UTC for Sep 21 8:00Am on UTC-9:00).
Also, the time part is significant when work items have a non integer duration (e.g. 1.5 days, 4 hours etc.).
You cannot remove or ignore the time part with the API, but obviously you can figure out the date from the returned times and build conditions for DateTime fields that would include whole days (e.g. "Start date is Sep 21" condition should be
Start Date >= Sep 21 0:00Am &&
Start Date < Sep 22 0:00Am. You may need to shift those times according to the organizational time zone).
Hope this helps,