func dayMatches(s *SpecSchedule, t time.Time) bool {
	var (
		domMatch bool = 1<<uint(t.Day())&s.Dom > 0
		dowMatch bool = 1<<uint(t.Weekday())&s.Dow > 0
	)
	if s.Dom&starBit > 0 || s.Dow&starBit > 0 {
		return domMatch && dowMatch
	}
	return domMatch || dowMatch
}
dayMatches returns true if the schedule's day-of-week and day-of-month
restrictions are satisfied by the given time.
package cron
import "time"
type SpecSchedule struct {
	Second, Minute, Hour, Dom, Month, Dow uint64
}
type bounds struct {
	min, max uint
	names    map[string]uint
}
var (
	seconds = bounds{0, 59, nil}
	minutes = bounds{0, 59, nil}
	hours   = bounds{0, 23, nil}
	dom     = bounds{1, 31, nil}
	months  = bounds{1, 12, map[string]uint{
		"jan": 1,
		"feb": 2,
		"mar": 3,
		"apr": 4,
		"may": 5,
		"jun": 6,
		"jul": 7,
		"aug": 8,
		"sep": 9,
		"oct": 10,
		"nov": 11,
		"dec": 12,
	}}
	dow = bounds{0, 6, map[string]uint{
		"sun": 0,
		"mon": 1,
		"tue": 2,
		"wed": 3,
		"thu": 4,
		"fri": 5,
		"sat": 6,
	}}
)
const (
	starBit = 1 << 63
)
func (s *SpecSchedule) Next(t time.Time) time.Time {
	t = t.Add(1*time.Second - time.Duration(t.Nanosecond())*time.Nanosecond)
	added := false
	yearLimit := t.Year() + 5
Wrap:
	if t.Year() > yearLimit {
		return time.Time{}
	}
	for 1<<uint(t.Month())&s.Month == 0 {
		if !added {
			added = true
			t = time.Date(t.Year(), t.Month(), 1, 0, 0, 0, 0, t.Location())
		}
		t = t.AddDate(0, 1, 0)
		if t.Month() == time.January {
			goto Wrap
		}
	}
	for !dayMatches(s, t) {
		if !added {
			added = true
			t = time.Date(t.Year(), t.Month(), t.Day(), 0, 0, 0, 0, t.Location())
		}
		t = t.AddDate(0, 0, 1)
		if t.Day() == 1 {
			goto Wrap
		}
	}
	for 1<<uint(t.Hour())&s.Hour == 0 {
		if !added {
			added = true
			t = time.Date(t.Year(), t.Month(), t.Day(), t.Hour(), 0, 0, 0, t.Location())
		}
		t = t.Add(1 * time.Hour)
		if t.Hour() == 0 {
			goto Wrap
		}
	}
	for 1<<uint(t.Minute())&s.Minute == 0 {
		if !added {
			added = true
			t = t.Truncate(time.Minute)
		}
		t = t.Add(1 * time.Minute)
		if t.Minute() == 0 {
			goto Wrap
		}
	}
	for 1<<uint(t.Second())&s.Second == 0 {
		if !added {
			added = true
			t = t.Truncate(time.Second)
		}
		t = t.Add(1 * time.Second)
		if t.Second() == 0 {
			goto Wrap
		}
	}
	return t
}