Arith-methods: Methods for Function Group Arith on mondate Objects
In mondate: Keep Track of Dates in Terms of Months

Arithmetic methods for class mondate. Includes three special "units between" methods.

signature(e1 = "mondate", e2 = "mondate")

Performs arithmetic operations on two mondates. The result will be numeric with attribute timeunits belonging to the slot of the first argument (with a warning if the two arguments have different timeunits). The only operation that would seem useful for two mondates is subtraction.

signature(e1 = "mondate", e2 = "numeric")

signature(e1 = "numeric", e2 = "mondate")

signature(e1 = "mondate", e2 = "array")

signature(e1 = "array", e2 = "mondate")

Performs arithmetic operations of a numeric on a mondate where the units of the numeric is taken from the timeunits slot of the mondate. The result will be a mondate with the same properties as the mondate in the function call.

The only operations that would seem useful for a mondate and a numeric are addition and subtraction.

Most of the time it is expected that timeunits="months" so that, for example, adding/subtracting a number to/from that mondate adds/subtracts that number of months. If the mondate's timeunits="years" then the perceived intention is to add/subtract that number of years. To accomplish that, the mondate's numeric value is divided by 12, the operation is performed, and the result converted to a mondate. If the mondate's timeunits="days", then the mondate's value is converted to the number of days since the start of the millennium, the operation is performed (e.g., the numeric number of days is added or subtracted), and the result converted to a mondate. (See the convert option of the as.numeric("mondate") function.)

+signature(e1 = "mondate", e2 = "difftime")

-signature(e1 = "mondate", e2 = "difftime")

Use a difftime object to add and subtract secs, days, weeks, months, and years to or from a mondate.

-signature(e1 = "mondate", e2 = "mondate")

Returns a difftime object equal to the signed number of units between e1 and e2, where "units" = timeunits(e1), with a warning if e1 and e2 have differing timeunits.

MonthsBetween(from = "mondate", to = "mondate")

Same as abs(from - to) in months.

YearsBetween(from = "mondate", to = "mondate")

Same as abs(from - to) in years, which is also the number of months between divided by 12.

DaysBetween(from = "mondate", to = "mondate")

Same as abs(from - to) in days, which is also the difference between the as.Date representation of from and to.

Arith

M <- mondate("1-1-2010") # will display in U.S. date format
M - 1:12 # the 1st of the month for all months in 2009
       # in reverse chronological order; inherits M's displayFormat

x <- mondate(matrix(12 * 1:4, 2, 2)) # 2x2 matrix of 2000-2003 year ends
x                                
y <- x + 12                          # one year later, also a matrix
y
y - x                      # 2x2 matrix of 12s, with an attribute ("months")
MonthsBetween(x, y)        # same, without the attribute
YearsBetween(x, y)
DaysBetween(x, y)

## Use difftime object to add, subtract secs, days, weeks, months, years
x <- mondate(1)            # January 31, 2000
y <- as.difftime(1, units = "days")
x + y
x - y
yw <- as.difftime(1, units = "weeks")
x + yw
x - yw
x + as.difftime(1, , "days")
x + as.difftime(86400, , "secs")

x <- mondate.ymd(2012, 2, 29)  # leap day
x + as.difftime(1, , "years")  # last day of February 2013, not a leap day
x - as.difftime(1, , "months") # not the last day of January