Option Explicit
Sub test()
Dim a, debut As Date, fin As Date, i As Long, n As Long, e, y
'Application.ScreenUpdating = False
With Sheets(1)
a = .Range("a2", .Range("a" & Rows.Count).End(xlUp)).Value
debut = Int(.Range("a2").Value)
fin = Int(.Range("a" & Rows.Count).End(xlUp).Value)
End With
With CreateObject("Scripting.Dictionary")
For i = debut To fin
.Item(i) = Empty
Next
For i = 2 To UBound(a, 1)
If .exists(Int(a(i, 1))) Then
.Item(Int(a(i, 1))) = True
End If
Next
For Each e In .keys
If Not IsEmpty(.Item(e)) Then .Remove e
Next
n = .Count: y = .keys
If n Then
With Sheets(1).Range("c1").Resize(n, 1)
.NumberFormat = "m/d/yyyy"
.FormulaLocal = Application.Transpose(y)
End With
End If
End With
'Application.ScreenUpdating = True
End Sub