Update Hourly Data count via Database Records

Question

I want to retrieve the records from the table in database, and from the TIMESTAMP, to put the counts in their appropriate hourly intervals. It works when say the first occurs AFTER 0730 hr and continues throughout the 24-hour. However, say when the first entry occurs in the midnight at 0430 hr, and continues through 0730 hr, then the counts gets wrong. how should I modify the attached code to cater for different cases?

thanks.