It's the end of an era (but how did they know?)

[Raynor]

After the making of the trees, the known counting of time began (the previous one is not known to the Eruhini); concerning the counting:

Originally Posted by Of the beginning of time and its reckonning, Annals of Aman, HoME X

Thereafter the Valar counted time by the ages of Valinor, whereof each age contained one hundred of the Years of the Valar

One year of the valar was roughly 10 Sun Years.