Paste Date And Time When User Selects From Dropdown

The code below adds the date to D:D once the user enters data in B:B and then adds the time when the user selects from a dropdown in C:C. When they have finished the task they enter anything into F:F and then it enters the end time in G:G.

This works well except for one flaw and one modification that I can't get to work. I have found that people retrospectively change the activity in C:C which causes the start time to change. I would like to avoid this hapenning if possible by putting some code in to stop this. Also I would like the times to be related to the push of a button on the sheet and therefore need different code.

Code:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Cells.Count > 1 Then Exit Sub
If Not Intersect(Target, Range("B2:B900")) Is Nothing Then
With Target(1, 3)
.Value = Date

[Code] ......