You always see rookies make the mistake of just adding up the individual time of all the tasks - and engineers are famously bad at making horribly under-scoped estimates - so projects estimated this way have typically went over time by a factor of three or so as I've come to learn...
https://www.purplemath.com/modules/bibleval.htm
https://answersingenesis.org/contradictions-in-the-bible/as-...