date(1)
Create dates stuff
Get year-month-day
Probably what you wanna have for prefixes, like always:
$ date +"%Y-%m-%d"
# => 2016-08-14
$ date -I
# => 2016-08-14
Get day-month-year
$ date +"%m-%d-%Y"
# => 08-14-2016
Get unix epoch time
Time since unix epoch, unix epoch, epoch time
$ date +'%s'
# => 1471126359
Get ISO date
ISO date can have varying specificity. It defaults to "date" but also accepts "hours", "minutes", "seconds" and "ns":
$ date -I
# => 2016-08-14
$ date --iso-8601=seconds
# => 2016-08-14T00:19:11+02:00