When retrieving more than a few appointments from a database it is especially useful to be able to
limit the query only to the visible timeframe of RadScheduler.
The VisibleRangeStart and VisibleRangeEnd properties are included to facilitate such
optimization.
Here they are used in conjunction with the parameters of SqlDataSource to avoid retrieving
redundant information. This also improves general performance and rendering times.
Please note that the select query is designed to always retrieve the recurring events.
This is necessary as we need to evaluate them in order to determine if they have occurrences
in the current time frame.