We have no specific dates for Isilmo, but as the younger child of Tar-Súrion, he must necessarily have been born after his elder sister Tar-Telperiën. We know that she was born in II 1320, so Isilmo would have been born shortly after that date. At least in earlier generations of his house, children tended to be born about eleven to thirteen years apart, so we might tentatively place Isilmo's birthdate at about II 1332. Tar-Telperiën went on to live for 411 years, so Isilmo's lifespan was presumably comparable. He certainly lived until the year II 1474, when his son Minastir was born.

The Isil in Isilmo's name definitely refers to the Moon, but the -mo ending is a little more difficult to parse. This is usually an 'agental' or occupational ending, so a ciryamo was a 'mariner', and the Vala Ulmo had a name meaning 'he who pours' or 'the pourer'. It is difficult to see how such an agental ending could apply to the Moon (unless perhaps Isilmo dedicated himself to studying it). The -mo ending could sometimes be used a more generalised name-ending, effectively meaning just 'person', and in this sense Isilmo would mean something as simple as 'Moon person' or 'Moon man'.