cpp/chrono/last spec

is an empty tag type that is used in conjunction with other calendar types to indicate the last thing in a sequence. Depending on context, it may indicate the last day of a month (as in, for last day of February 2018, i.e., 2018-02-28) or the last day of the week in a month (as in , for last Sunday of February 2018, i.e., 2018-02-25).