It is not unusual for some/many activities to have duration shorter than a whole day, say for example concrete pouring.  If you cannot have duration shorter than a work day then why allow scheduling by the hour and minutes?  The workaround makes no sense in many scenarios.

Concrete pouring is usually required to be completed within the same work day, if not then shall be scheduled for next available whole day.

Frequently concrete pouring of slabs are allowed to be scheduled to start at earlier time of day and finish latter time of day than other activities. 

There is no reason why you cannot have; activities starting at different time of day, activities with remaining durations shorter than a whole day or required to be performed within the same day, the software shall always give the expected start dates. 

P6 is notorious for flaws in the calculations of dates and float values under multiple calendars.  

PRIMAVERA P6 – FLOAT ON CRITICAL ACTIVITIES?

While it’s quite legitimate to use multiple calendars in any schedule, it can present problems if we attempt to mix those calendars within the logic of the schedule. That is to say activities on one calendar should not be linked to activities on a different calendar if at all possible. Doing so can generate some very perplexing results in not only the dates, but the critical path.

Oracle P6: Critical Path Identification Disorder

One of the most frequently used features of P6 is the capability of using different calendars to schedule different activities of the same project. However, the use of multiple calendars affects the Total Float calculations and accordingly the Critical Path identification.

I have talked to Oracle representatives in several occasions and created Enhancement Requests in Oracle, but unfortunately it seems that Oracle does not have any plans to enhance its P6’s scheduling algorithm in the near future. P6 users should not assume that P6 will always produce a reliable critical path even if the poor scheduling practices are avoided.

P6 -What are critical path activities?

When resource leveling P6 longest path does not works.

The longest path is broken when activities are no longer driven by relationships; that is, when activity dates are driven by constraints or resource leveling. 

This seems to be another BUG.

your TIMES are incorrect

your data date time should match your calendar TIME.

You durations may be in fractions of days not whole days

here is the solution

your TIMES are not correct and or your remaining durations are in fractions of days not whole days,  Edit users preference,  Click on the Dates Tab , Click show minutes (12 hour) Click on the Time Units Tab, Next under the Duration Format Unit of Time Decimals change this value to 2 Decimals, If you see any Original Durations and or Remaining Durations with decimals round them up or down and change them to whole numbers. Go to the project directory click on dates make sure your project start TIME to 8:00 am or the earliest time in your calendar. Change the data date TIME to 8:00 am or the earliest time in your calendar. Change the Project Must Finish by date TIME to 5:00 pm or the LATEST times in your calendars. If you have any start constraints TIME to 8:00 am or the earliest time in your calendar.If you have any finish constraints TIME to 5:00 pm or the LATEST times in your calendars. Any activities that are inprogress make sure that the start TIME is  8:00 am or the earliest time in your calendar begin used.

assuming this is happening on an updated schedule and not a baseline schedule, it is due to the percent complete assigned to the activity and the fact that P6 durations are based on hours , for example you have an activity that is (2) days long  on a 8 hour calendar (16 hour duration) and you update it to 75% complete (or 4 hours remaining) that activity will comlplete 4 hours the next day allowing the successor to start on the same day or (4) working hours into that day