Validate if the selected date and time is free to choose.

The user selects a date and a time (datetime) in a form field.
The user will then receive a message if this time is available or occupied. If it is busy, an error will occur.

The idea is that when a user has filled in a start time and an end time, this time cannot be used by anyone else.

I imagine that the database table should contain a start time (datetime) and an end time. And I am now wondering if there is any way to validate this.

You can create a recordset to check if that time is available and then validate if that recordset has any results.

Can you explain a little more clearly? Do you mean that the recordset should contain the columns or should I filter it somehow?

Your recordset would look to see if there is already a record that has a start time that is after the requested start time that is between the requested start and end time, or an end time that is between the requested start time and end time. Then you can check the TotalRows of the recordset to see if there is a conflict based on if that recordset has a result or not.