as_xts is a wrapper for xts::as.xts
that includes a date_col argument. When date_col is specified,
the date column is used as row names during coercion to xts class. The
date column must be in a date format (i.e. date class).
as_xts_ evaluates the date_col using Non-Standard Evaluation
(NSE). See vignette("nse") for details.
It is possible to coerce non-data.frame-like objects including
zoo, timeSeries, ts, and irts objects.
There is no need to specify the date_col argument.