- period
A character specification used for time-based grouping. The
general format to use is "frequency period"
where frequency is a number
like 1 or 2, and period is an interval like weekly or yearly. There must be
a space between the two.
Note that you can pass the specification in a flexible way:
This shorthand is available for year, quarter, month, day, hour, minute,
second, millisecond and microsecond periodicities.
Additionally, you have the option of passing in a vector of dates to
use as custom and more flexible boundaries.