It is important to track the reason behind any major changes to the cost of a project. Project costs can be affected by time changes, scope changes, price escalation factors and many other reasons.
Recommended sources: Project management timelines, budget documents, quantity surveyor or other technical adviser reports.
Timing of data input: Ongoing throughout project.
Issue: The table is a fairly high-level snapshot of changes and their impact. The comments section is where explanations can be provided.
Issue: It may not be possible to translate time changes into a $ impact (e.g. additional project management costs may be identifiable but not opportunity costs). In this case note the explanation for the major time changes in the table or comments section