No dateTime.plusMonths(), dateTime.plusYears() etc.

[johnnyfivedev]

Where is only plus(Integer aNumYears, Integer aNumMonths, Integer aNumDays, Integer aNumHours, Integer aNumMinutes, Integer aNumSeconds, Integer aNumNanoseconds, DayOverflow aDayOverflow) and plusDays(Integer aNumDays) methods in DateTime class. Why there's no plusMonths(Integer aNumMonths), plusYears(Integer aNumYears) etc? Same for the minus.