Examples: The Yes/No field in Access does not support Null as other databases do.
To simulate a real Yes/No/Null data type, use a Number field (size Integer) with this rule.
This article explains how to use validation rules, and concludes with some thought provoking on when to use them.
When you select a field in table design, you see its Validation Rule property in the lower pane.
The field names are self-explanatory in that the Start Date refers to the date at which a conference started or starts and the End Date fields refers to the either a future date when the conference will end or when it ended some time in the past.