In BioStar 2 T&A the Rebuild function is a complete different concept compared to BioStar 1.
In BioStar 1 it was a feature to delete events edited by the administrator.
In BioStar 2 it is the feature that updates the report up to the current time.
If you click Update Report and do not select Rebuild Time Card, the last report you built before will be loaded.
Hence if you want to see the up to date report you will always have to check Rebuild Time Card before clicking Update Report.
In BioStar 2.7, a new optional feature will be added that will automatically build the report at a specified time of the day.