A function like
TIMEVALUE("8:00")) might work for this use case but the considerations document linked below has this note, so formulas may not be an option here:
Time triggers can’t reference the following:
DATE or DATETIME fields containing automatically derived functions, such as TODAY or NOW.
If you need this requirement to scale reliably, I would consider running a schedulable instead. Time-based workflow rules are constrained to a certain amount per hour and if you exceed that limit (due to the cumulative number of other time-based workflow rules) your outcomes can get pushed forward to the next hour or beyond. Refer to Considerations for Time-Dependent Actions and Time Triggers for details. Relevant limitation quoted below:
Salesforce limits the number of time triggers an organization can execute per hour. If an organization exceeds the limits for its Edition, Salesforce defers the execution of the additional time triggers to the next hour. For example, if an Unlimited Edition organization has 1,200 time triggers scheduled to execute between 4:00 PM and 5:00 PM, Salesforce processes 1,000 time triggers between 4:00 PM and 5:00 PM and the remaining 200 time triggers between 5:00 PM and 6:00 PM.
Something you may want to look at are Business Hours. Using Business Hours you could have your functionality run at "open of business" each day, skipping weekends and holidays.