I decided to just outright ask http://www.gamesas.com/user/365941-gstaff/ about the vagueness concerning just when Serana was put away. Here's what he said:

My initial PM:

Here's GStaff's reply:

So.... She got put away during the Second Era Interregnum.

As far as I'm concerned, Word of God is always optional when it appears in TES... but luckily they got what most people figure is the right answer anyway, hah. Thanks for the info.

With this answer, she might even be using sarcasm - "Cyrodiil is the seat of an Empire? Pfff, I saw a score of barbarian kings rise to the Ruby Throne and fall in the same month." Empire wasn't so much absent from the Interregnum as crude, ephemeral and continuously collapsing.

I opened this thread expecting to read another "What era is Serana from?" question and was pleasantly surprised.

I think it's been a while since we've gotten a Word of God response from Bethesda on... almost anything, hasn't it?

The one problem with this is that line about the Dwemer when entering a Dwemer ruin. If she's from the Second Era, she must know about the disappearance of the Dwemer, no?

typo me thinks