POSIX says about `touch -t [[CC]YY]MMDDhhmm[.SS]`:

> If SS is not given a value, it is assumed to be zero.

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/touch.html

Busybox touch does not set it to zero, instead it uses the localtime's seconds.

To reproduce: 

  TZ=UTC strace busybox touch -t 197001010000 /tmp/foo

  ...
  utimensat(AT_FDCWD, "/tmp/foo", [{23, 0}, {23, 0}], 0) = 0
  ...


note that utimesat(2) is called with the seconds part to 23.

Doing the same with the .ss specified will correctly set the seconds part to 0:

  TZ=UTC strace busybox touch -t 197001010000.00 /tmp/foo

  ...
  utimensat(AT_FDCWD, "/tmp/foo", [{0, 0}, {0, 0}], 0) = 0
  ...


The reason is that the time_t struct is initialized with localtime instead of zero:
https://git.busybox.net/busybox/tree/coreutils/touch.c#n155