U2 couldn't have existed in Roman Times. Besides the whole not having electricity thing, they couldn't have had their name. There was no letter "U" in Roman Latin, and they didn't have Arabic numerals. So, instead, they would have to be VII (as "V" was used for "U" and "II" is the Roman numeral for "2"). But then everyone would just think they were called "Seven" (as "VII" is the Roman numeral for 7), kind of like confusion of the name of the band in That Thing You Do!
If this isn't proof I should be committed, I don't know what is.