consistently use time_t and size_t

Hi there,

when compiling this module for Win32 (Microsoft C++ compiler), a couple of warnings pop up due to inconsistent usage of time_t and size_t. This PR fixes the warnings (and makes sure push.cpp will work beyond 2038 ;-)

Tested on MSVC 2013 (32 + 64 bit) and GCC 4.7 (x86_64).

Thanks!