For all use cases, I can think of, SRFI 226 needs only two procedures
that deal with times:

(time? OBJ)
Returns #t if OBJ is a time value representing a point in (proper)
time, #f otherwise.

(nanoseconds-from-now N)
Returns a time value representing N nanoseconds from the time of
invoking this procedure.

For convenience, one can add a third procedure:

(seconds-from-now X)
Returns a time value representing X seconds from the time of invoking
this procedure.

This very minimal interface should be compatible with any high-level
or extended API.

Do you like the names "nanoseconds-from-now" and "seconds-from-now",
or have you thought of better names?

Thanks,

Marc