PerformanceAnalytics Figure 13

To clarify, the format "YYYY-MM-DD" isn't an option for formatting dates in
Excel if the locale is set to English (U.S.). If you switch the locale to
English (U.K.), or probably any other country, this format option becomes
available. 
Regards

Most recent versions of Excel offer an 'ISO-8601' or a 
YYY=MM=DD' format, which os correct.  The ommission of that format in
the 'English(United States)' locale hardly counts as a problem for R.
All versions of Excel for at least a decade have also offered a 'custom'
format.

There are more reasons than could be conveniently enumerated to not use
Excel for quantitative finance.

But this is an R list, not an Excel list, so I'll stop there.  

Regards,

    - Brian