It works just like one-member structs, in that it can be treated as a single value, passed and returned by value when calling functions, and keeps you from accidentally mixing different kinds of values. Having the "epoch" field along for the ride just means you can add some additional smarts.
So yes, like I was thinking; a tagged union - the epoch field determines the interpretation of the val field. The full application of "wrap values in single element structs for better static guarantees" would be to have a different time struct for every epoch. This is a (possibly quite useful) step back from that, since C's lack of polymorphism would mean a need to implement every time function for epoch even when the logic is the same.
I guess that makes sense now that you explain it. Conceptually, each epoch value results in a different type for the other field, meaning it works like a union, even though it's actually implemented using the same primitive type for each.
