date_t replacements not statically cached

In date_t() there is a call to date_t_strings(). date_t_strings() proceeds to call t() a dozen times and do manipulations on that data. t(), if locale is enabled, is not a fast function. This data should be statically cached.

In fact, it is statically cached in date_t(). Or rather, it should be. There's even a commented out static variable in there. I don't know why it's commented out.

The attached patch removes the comment to re-enable static caching. On the site I'm working on that results in a savings of about 14000 (yes, fourteen thousand) function calls and around 600 ms of execution time.

Hence the "Quick Fix, Performance, Critical." :-)