accepts a custom format string to return a daystr can be either a prefix, a delta, or a prefix w/ a delta if no prefix is given, "today" is assumed examples: today-2d, bow, bom+1m-1d (that's end of the month), 2024-04-01+1w prefixes: yyyy-mm-dd today yesterday bom (beginning of month) bow (beginning of week -- sunday) deltas: +[n]d, -[n]d (e.g.