The day_periods table contains rules for naming periods of time throughout
a day. There are 519 rows that comprise a day period name
and rule for a locale. There may be only two rows associated with a locale
but many more if a locale has many names for periods of a day. The following
columns are included:
locale(character)period(character)from(character)to(character)at(character)
The period value provides an identifier for the period of time. For a given
locale there may typically be "afternoon1" and "evening1" period
identifiers. Some may have quite a few periods defined (perhaps with
"morning1" and "morning2" rules). A period is either a block of time
defined by the from and to columns, or, a set time (like "noon" and
"midnight") found in the at column. The period values are typically
obtained from this dataset in order to obtain localized text from
the dates and dates_generic datasets (within the dayperiods_* columns).