-f freshen existing files, i.e., extract only those files that already exist on disk and that are
newer than the disk copies. By default unzip queries before overwriting, but the -o option may be
used to suppress the queries. Note that under many operating systems, the TZ (timezone)
environment variable must be set correctly in order for -f and -u to work properly (under Unix the
variable is usually set automatically). The reasons for this are somewhat subtle but have to do
with the differences between DOS-format file times (always local time) and Unix-format times
(always in GMT/UTC) and the necessity to compare the two. A typical TZ value is ``PST8PDT'' (US
Pacific time with automatic adjustment for Daylight Savings Time or ``summer time'').