func (s Span) Before(t time.Time) bool {
	return s.end.Before(t)
}
Before reports whether s ends before t.
package timespan
import (
	"time"
)
type Span struct {
	start, end time.Time
}
func New(t time.Time, d time.Duration) Span {
	start := t
	end := t.Add(d)
	if end.Before(t) {
		start, end = end, start
	}
	return Span{
		start: start,
		end:   end,
	}
}
func (s Span) Start() time.Time {
	return s.start
}
func (s Span) End() time.Time {
	return s.end
}
func (s Span) Duration() time.Duration {
	return s.end.Sub(s.start)
}
func (s Span) After(t time.Time) bool {
	return s.start.After(t)
}