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
be formatted according to its tclass() (e.g. Date, POSIXct, timeDate,
yearmon, etc.).
The 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.