func (ps periodicSchedule) Next(t time.Time) time.Time {
	return t.Truncate(time.Second).Add(ps.period)
}
Next adds time t to underlying period, truncates up to unit of seconds.
package gron
import (
	"errors"
	"time"
	"github.com/roylee0704/gron/xtime"
)
type Schedule interface {
	Next(t time.Time) time.Time
}
type AtSchedule interface {
	At(t string) Schedule
	Schedule
}
func Every(p time.Duration) AtSchedule {
	if p < time.Second {
		p = xtime.Second
	}
	p = p - time.Duration(p.Nanoseconds())%time.Second  
	return &periodicSchedule{
		period: p,
	}
}
type periodicSchedule struct {
	period time.Duration
}