Depends on your supplement....
If you're in the southern region and you can prove you were scheduled that day, it looks to me like you're guaranteed 8 hours.
LETTER OF UNDERSTANDING ARTICLE 49, SECTION 7 Temporary Cover Drivers Rate of Pay April 21, 2009
If an employee on the preload operation, who is a TCD and is scheduled to cover for a full-time Service Provider, then said employee would be guaranteed eight (8) hours per day, and would be considered to be the junior employee in that classification for purposes of extra work opportunities for that day.
https://teamster.org/sites/teamster.org/files/southern-region-supplement-07172014_0.pdf