That's the whole point of the site. If I'm trying to figure out what time it is for my co-worker in Canberra Australia, I don't want to have to Google what time zone they are in (Australian Eastern Daylight Time). Just put the city in and it does the math for you.
I mean, other sites[0] that I've used seem to have a list of major cities (not Google Maps autocomplete)
I'm talking about the official "tz" database, which contains a large list of cities and geographic coordinates. So if you want to autodetect, you just need approximate lon/lat and then grab the closest tz city.