Count number of days from today

Welcome to the forum.

If the task is complete, which cell will show the complete date?

=IF(C1="Not Completed",(TODAY()-B1)+1,(cell_with_complete_date-B1)+1)

EDIT:

the date should should stop counting on the day that the status showed completed.

This cannot be done without VBA. Just add a column for the completed date - you can set data validation to prompt its being filled in once C1 changes to Completed.

Put this in worksheet module (Right click on tab's name, ViewCode then paste this code into)

PHP Code: 

Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address(0, 0) <> "C1" Then Exit Sub
With Range("D1")
    Select Case LCase(Range("C1").Value)
        Case "not completed"
            .Formula = "= today() - B1 + 1"
        Case "completed"
            .Value = .Value
    End Select
End With
End Sub 


I have moved your thread. In future, please post in the VBA section for code queries.

OK. Like this:

PHP Code: 

Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column = 3 And Target.Row > 1 Then ' trigger change from C2 downwards
    With Target.Offset(0, 1) 'next cell in column D
        Select Case LCase(Target.Value)
            Case "not completed"
                .Formula = "= today() - " & Target.Offset(0, -1).Address & "+ 1"
            Case "completed"
                .Value = .Value
        End Select
    End With
End If
End Sub