Maintenance for the week of March 16:
• PC/Mac: No maintenance – March 16
• ESO Store and Account System for maintenance – March 18, 6:00AM EDT (10:00 UTC) - 4:00PM EDT (20:00 UTC)
I've used both - I prefer UserID but whichever you choose, be SURE to spell it right! In game, the capital i (I) and the lower case L (l) are as identical as they are in text here, and that can be a real problem, as are non-English characters.