Relative
My Friday Thought: Context is king: If you're using your experience as an estimation then it needs to be for a similar thing. Saying we did [something] within [time] and using that as the estimate for all, when there have added complexity & have to follow a different path doesn't help.
Re-platform over migration.
Often I think the difference of these are completely missed. I re-platform is a lift and shift a move from one place to another - minimum user changes, less complexity and often easy (relatively) to deliver. A migration is not this, it has significant user changes, added complexity and often can't be delivered by one team, but needs much co-ordination. Comparing the two is unfair.