Learn Danish: De har kendt hinanden i et års tid. - They have known each other for about a year.

In Danish, adding s tid to a unit of time makes it an approximation. While et år means exactly one year, adding the genitive s followed by tid (literally "a year's time") translates idiomatically to "roughly a year" or "a year or so". You will also hear this with other time periods, like en uges tid (about a week) or en måneds tid (about a month).

Why use the preposition i to mean "for" in this sentence?

When expressing the duration of an action (how long something has been happening), Danish uses i. English speakers are often tempted to use for, but in Danish, for in time expressions is usually reserved for things like for et år siden (a year ago). To say "for a year" as a duration, it must be i et år or, as in this sentence, i et års tid.

Would it be understood if I used vidst instead of kendt here?

It would sound very wrong to a Dane. Even at an advanced level, it is easy to mix up the two verbs for "to know". You must use at kende (past participle kendt) when talking about being acquainted with people, places, or things. The other verb, at vide (past participle vidst), is strictly used for knowing facts or information.

Does the word hinanden change if we are talking about a group of three or more people?