While the sui po and the yue po are vigorously used in date selection for the avoidance of bad days to do stuff, the ri po (日破) enables a date selection specialist to identify inauspicious hours of the day so that they can be avoided to undertake important activities.
Also known as the day breaker, each day would have a two-hour block where the ri po takes position.
This because each day is governed by the 12 earthly branches. They therefore consist of 2 hours each and they take turns in ruling the energies of the day.
The Hsia calendar can be referred to to identify which earthly branch rules for each specific day.
Once the earthly branch of day is determined, the branch that is opposite it would be the day breaker. The hour of conflict can then be identified.
For example, if it is revealed that the earthly branch of the day is E3, then the ri po would be E9. Thus, the hour of between 1500 to 1700 would be the hours to avoid.
Legend of earthly branches can be found here.
Table of ri po (day breaker)
It must be said that most people find that selecting a date to conduct certain activities sufficient for day selection.
Using the day breaker method for hour selection is usually only applied to very specific events.