Consider the following code:
DateString[{2000, 1, 1, 0, 0, 0}, {"Year", " ", "Month", " ", "Day",
" ", "SecondExact", " "}]
yields
2000 01 01 00.000
but
DateString[{2000,1,1,0,0,0},{"Year"," ","Month"," ","Day"," ","MinuteExact"," "}]
and
DateString[{2000,1,1,0,0,0},{"Year"," ","Month"," ","Day"," ","HourExact"," "}]
yield
2000 01 01 {0., QuantityMagnitude[Minute]}
and
2000 01 01 {0., QuantityMagnitude[Hour]}
respectively.
It does not seem like the units should be present in the last two cases for Minute and Hour.