Comment 3 for bug 51106

its also in coreutils 6.0 so it's upstream.
If you do:

date -d '11:40 8 minutes'

(so leave out the '+'), it gives the right answer: 11:48
11:40 60 seconds gives 11:41 (correct)
11:40 + 1 day gives correct answer
11:40 1 day gives correct anser.

With day, week, year, you can add the plus sign, (or minus sign) doesn't make any difference.
With seconds, minutes and hours, date does strange things when you add a plus or minus sign.

This is probably an upstream bug.