Trying to get query to work the way I want it to work

I have a form with three fields... a start date, an end date, and an employee combobox.  Then I have a query that has this syntax in the Date field:

Between [Forms]![frmTimeReportingHeader]![txtStartDate] And [Forms]![frmTimeReportingHeader]![txtEndDate]

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But in the employee field I either want the record narrowed to the selection on the employee combobox.  This syntax works fine:

[Forms]![frmTimeReportingHeader]![cboEmployee]

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But I cannot figure out what the employee syntax should be if I want all employees.