Variable(s) that define unique time indices, used in conjunction
with the helper id(). If a univariate time series (without an explicit key),
simply call id().
index
A bare (or unquoted) variable to specify the time index variable.
regular
Regular time interval (TRUE) or irregular (FALSE). The
interval is determined by the greatest common divisor of index column, if TRUE.
validate
TRUE suggests to verify that each key or each combination
of key variables leads to unique time indices (i.e. a valid tsibble). If you
are sure that it's a valid input, specify FALSE to skip the checks.