Have you enabled the “Use hours” option in the calendar settings ?
When you are setting availability with hours are you checking both option for hours, in the availability form ?
“Change hours definitions (changing the definitions will overwrite any previous hours data)”
“Set default hours values for this day(s). This will overwrite any existing data.”
During the winter holidays, 21 December 2015 – 4 January 2016, answers will be delayed.