macro msg box

You could do it with this :-

Sub Get_Todays_Date()
Dim DateInput As String
DateInput = Application.InputBox("Please enter today's date")
If DateValue(DateInput) = Date Then
[A1].Value = DateValue(DateInput)
Else
MsgBox "You must only enter today's date"
End If
End Sub

But why ask for the current date to be input?
Why not just have the macro enter the current date in A1 with a single line of code:-
[A1].value = Date

Correction .......

The macro should be :-

Sub Get_Todays_Date()
Dim DateInput As String
DateInput = Application.InputBox("Please enter today's date")
If IsDate(DateInput) Then
If DateValue(DateInput) = Date Then
[A1].Value = DateValue(DateInput)
End If
Else
MsgBox "You must only enter today's date"
End If
End Sub