What version of "date(1)" are you talking about?

Sorry, OP didn't mention anything about his/her env.

"-d STRING" doesn't seem to report seconds since epoch, but date +%s does

"-d STRING" will be used for passing custom time, otherwise you'll be formatting the current time ;-)

e.g.:

$ date -d '1987-12-18 12:13:14' -u '+%s'
566827994
$ date -d '1987-12-18' -u '+%s'
566784000
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