Inflexible was ordered in February 1777, laid down that April, and launched on 7 March 1780. Launch provides the date used to distinguish the ship's age from her first active commission. A hull could spend years in ordinary after launch. The hypothetical example is a ship launched in 1760 that did not commission until 1772.
Notable Point · 17:59-18:36
Why a ship's launch date differs from her first commission
Inflexible's construction dates introduce the distinction between a hull's launch and entry into commissioned service.