This holds onto information about how a particular locale formats dates. It
contains mostly strings, e.g. what the names of months or weekdays are,
but also indicates things like the first day of the week. We expect the data
for instances of these to be generated out of ICU or a similar reference
source. This is used in conjunction with the date_time_patterns, which
defines for a particular locale the different named formats that will
make use of this data.