The R7RS draft fixes a longstanding hole by adding the simplest possible time function:
current-second, which returns the time since 1970 in seconds.
Unfortunately, it specifies that the timescale used is TAI. This is overspecification. The advantage of TAI over POSIX time is linearity (POSIX time jumps backward at leap seconds, so it's ambiguous, and time arithmetic gives incorrect results), but R7RS acknowledges that most implementations will compute it by simply subtracting a constant from their system clock. In addition to not being correct TAI and being gratuitously incompatible with POSIX, the resulting timescale remains ambiguous and nonlinear, so it has no advantage over POSIX time. Why specify a feature when you know it won't work?
I agree that the POSIX timescale is messed up, but this is not something languages can fix. As long as time is provided by the operating system, the choice of timescale is up to the operating system.
Also, time standards have historically been replaced fairly often, so Scheme risks specifying an obsolete standard which no one else uses. It's safer to simply leave the timescale and epoch unspecified, and let
current-second be the system clock, whatever it is.