This function is typically used to discard information in non-quarter month.
I.e., data is only kept in January, April, July and December and otherwise set
to NA. In combination with m_to_q
this function is useful to
turn monthly series into quarterly series by letting the quarter month values
represent the entire quarter. This can be useful when data was interpolated
because of mixing data of different frequencies and needs to be converted
back to a regular, quarterly time series.
Usage
set_month_to_NA(series, keep_month = c(1, 4, 7, 10))