The format is rather unusual.

The time is 16 bits: 5 bits for the hour, 6 bits for the minutes, and 5
bits for the seconds. 5 bits only allows 32 values (0-31) so the number
must be doubled, meaning that the time is only accurate to the nearest
two seconds. Also, the JDK rounds this down by subtracting one. The
doubling and rounding is performed by shifting one place to the left
and masking off the right-most bit respectively.

The date is 16 bits: 7 bits for the year, 4 bits for the month, and 5
bits for the day. The year is from 1980, i.e. the earliest date that
can be represented is 1980-01-01.

See http://mindprod.com/jgloss/zip.html for more details of the format.

Fixes gh-2826