He's singing about "time" (singular noun).

It's possible to use the singular form if the person is talking about the amount as one single thing. This is common with money, time, and distance. Five hundred dollars is a lot to spend on a pair of shoes. Twenty years have passed since we last met. Twenty-five miles was too much for many of the runners.

Songwriters in general take liberties with grammar.

When referring to a grouping, you can use the singular. In this song, the target of the verb is seven, not years. This can be a single amount, seven, and can use a singular verb. Measurements are usually referred to as a total group, and use the singular. This is rarely important except when the group has properties that the single units do not. Example: 7 is the correct amount. - The correct amount is 7. Any single unit would be incorrect. 7 are the correct amount. - Of the amounts, 7 of them are the correct amount. There may be other amounts which are not the correct size or number. The (American) football field is 100 yards long. Ten yards is a first down. - Any 10 yards together make a first down. This is correct. Ten yards are a first down. - 10 of the 100 yards are each a first down. The other 90 are not. This is not correct. So in this song, the author could have used either "has" or "have".