use ts for tses, including in schedule

pttodo/schedule.go

@@ -86,5 +86,5 @@ type scheduleDue int64
 var scheduleDuePattern = regexp.MustCompile(`^[0-9]+$`)
 
 func (due scheduleDue) next(time.Time) (time.Time, error) {
-	return time.Unix(int64(due), 0), nil
+	return TS(due).time(), nil
 }

pttodo/ts.go

@@ -7,6 +7,10 @@ import (
 
 type TS int64
 
+func (ts TS) time() time.Time {
+	return time.Unix(int64(ts), 0)
+}
+
 func (ts TS) MarshalJSON() ([]byte, error) {
 	v, err := ts.MarshalYAML()
 	if err != nil {