Really, they're twins. They were commissioned on the same day.

Actually that depends how you measure their age.

Both were commissioned on April 19th, 1915 (By this measurement they're the same age)

Haruna's keel was laid on March 16th, 1912; Kirishima's keel was laid on March 17th, 1912 (this appears to be how Hisahiko is measuring their age, Haruna is 1 day older)

Haruna was completed on December 14th, 1913; Kirishima was completed on December 1st, 1913. (So by this metric, Kirishima is the elder)

seikenshin said:

Really, they're twins. They were commissioned on the same day.

It’s complicated

Dranikos said:

Actually that depends how you measure their age.

Both were commissioned on April 19th, 1915 (By this measurement they're the same age)

Haruna's keel was laid on March 16th, 1912; Kirishima's keel was laid on March 17th, 1912 (this appears to be how Hisahiko is measuring their age, Haruna is 1 day older)

Haruna was completed on December 14th, 1913; Kirishima was completed on December 1st, 1913. (So by this metric, Kirishima is the elder)

So...
Date commissioned = Start trying to have a kid.
Date keel is laid = Date of conception/beginning of pregnancy.
Date completed = Date of birth.
Alternatively, date launched = Date of birth.

Owen_Self said:

So...
Date commissioned = Start trying to have a kid.
Date keel is laid = Date of conception/beginning of pregnancy.
Date completed = Date of birth.
Alternatively, date launched = Date of birth.

Commissioning comes after launching, which comes after laying down the keel. The commissioning refers to commissioning a ship INTO active service (like commissioning an officer), and not commissioning a purchase order.

A launched ship is usually just a floating hulk with barely any equipment. Most ship systems (and weapons) aren't even installed yet. They're installed and rigorously tested during the period of launching and commissioning, as sea trials. The crew is often also trained during this time as well.