Fix add/sub reported bug (Reported By @Alireza-Jamali).[https://github.com//issues/81]
Add new adds methods: addWeeks,addDays,addMonths,addYears,addHours,addMinutes,....
Add new subs methods: subsWeeks,subsDays,subsMonths,subsYears,subsHours,subsMinutes,....
Increase test coverage.