zkSysTime.h is a portable wrapper for the Unix 98 header <sys/time.h>. It is part of the libzk library because <sys/time.h> doesn't exist in Standard C.

zkSysTime.h - Supported Types from <sys/time.h>
Type Description
tv_sec_t Integral type expressing a number of elapsed seconds since 00:00 Coordinated Universal Time (UTC), January 1, 1970.
struct timeval, with members:  
tv_sec_t    tv_sec
The number of elapsed seconds since 00:00 Coordinated Universal Time (UTC), January 1, 1970.
suseconds_t tv_usec
The number of elapsed microseconds since the time given in tv_sec.

zkSysTime.h - Deprecated Functions from <sys/time.h>
Don't Call This Instead, Call This Reason
gettimeofday zkGettimeofday Not in Standard C.