Time difference in alert packets are reversed

See https://git.ligo.org/emfollow/gwcelery/-/blob/721e23fc470d69d6c36f39e365197e4aaa4fb46d/gwcelery/tasks/alerts.py#L200-201

This should be the other way around, where a positive number indicates how long after an external event is from t_0 while a negative number should be before.