The date of Valandil's birth appears only in The History of Middle-earth volume XII, The Peoples of Middle-earth. It cannot therefore be considered completely reliable. That said, it does match well with the established fact that he reached his majority in III 10, when he would have been twenty-one years old.
The placing of stress in Valandil's name is open to question. According to the rules presented in Appendix E of The Lord of the Rings, the stress should be placed on the second syllable of his name if it is followed by two consonants. In English, this appears to be the case: the second syllable is followed by n and d. In Elvish scripts, though, nd is represented by a single letter, ando, so this rule would not seem to apply. For this reason, though the name is shown with the stress on the first syllable above, it should be noted that the more natural vala'ndeel is probably a valid alternative.