Date-Time Pivot chart

Date-Time Pivot chart

Hi, my boss want to see in a grapich, how many "Altas Voluntarias Report" are there between a specific time, so how can i do to create a pivot chart to see these dates in a grapich? 

These are the date ranges:

6:00 am - 10:00 am
10:00 am - 2:00 pm
2:00 pm - 6:00 pm
6:00 pm - 10:00 pm
10:00 pm - 6:00 am

It's no use saying that on my form I have a date-time field