Valid values for the value argument are the same as specified in the
Details section of strptime.
An xts object's tformat is NULL by default, so the index will be
formatted according to its tclass (e.g. Date,
POSIXct, timeDate, yearmon, etc.).
tformat only changes how the index is printed and how the row
names are formatted when xts objects are converted to other classes (e.g.
matrix or data.frame. It does not affect the internal index
in any way.