Shift months one or two years back
These functions are used to specify months from previous years for
the design matrix. .(1) is the same as -1, and
specifies previous year's January, whereas ..(1) signifies
January of the year prior to the last year.
.(x) ..(x)
x |
numeric identifier (of length 1) for the months (1 for January, ..., 12 for December) |
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