If you want Cacti to export static images on an hourly basis, put the minutes of the hour when to do that. Cacti assumes that you run the data gathering script every 5 minutes, so it will round your value to the one closest to its runtime. For instance, 43 would equal 40 minutes past the hour.
If you want Cacti to export static images on an daily basis, put here the time when to do that. Cacti assumes that you run the data gathering script every 5 minutes, so it will round your value to the one closest to its runtime. For instance, 21:23 would equal 20 minutes after 9 PM.