When the format is yyyy/mm/dd a simple string sort will work.

Code:
Private Sub Form_Load()
    With Me.MSFlexGrid1
        .Rows = 5
        .TextMatrix(1, 1) = "2007/06/23"
        .TextMatrix(2, 1) = "2007/01/05"
        .TextMatrix(3, 1) = "2007/06/03"
        .TextMatrix(4, 1) = "2007/01/01"
        .Sort = flexSortStringAscending
    End With
End Sub