Time is not a human construct. Time exists whether we do or not. The labels we choose are constructs, but the underlying passage of time still occurs, whatever we label it.

Further some things, such as a year, are defined by external factors, namely the Earths orbit around the Sun. This does not change with the existence of humans. It is a known quantity. The orbital periods of other planets are indeed different, but thats not what is referred to by "a year", which is why you have to qualify in circumstances where thats what you want to talk about.