Inconsistencies between different gps and unix time conversion implementation

[boyang9602]

Describe the bug There are multiple implementations of GPS and UNIX time conversion in Apollo. The first is in https://github.com/ApolloAuto/apollo/blob/master/modules/common/util/time_util.h, the second is in https://github.com/ApolloAuto/apollo/blob/master/modules/drivers/gnss/util/time_conversion.h, the third is in https://github.com/ApolloAuto/apollo/blob/master/cyber/common/time_conversion.h, the last is in https://github.com/ApolloAuto/apollo/blob/master/modules/localization/msf/common/util/time_conversion.h.

To Reproduce Steps to reproduce the behavior: I compared the first three implementations with GPS time 1001696182.543048

#include <iostream>
#include "time_util.h" // https://github.com/ApolloAuto/apollo/blob/master/modules/common/util/time_util.h
#include "time_conversion.h" // https://github.com/ApolloAuto/apollo/blob/master/modules/drivers/gnss/util/time_conversion.h
#include "time_conversion_new.h" // https://github.com/ApolloAuto/apollo/blob/master/cyber/common/time_conversion.h

int main() {
    double gps_time = 1001696182.543048;

    // https://github.com/ApolloAuto/apollo/blob/master/modules/common/util/time_util.h
    double unix_time = TimeUtil::Gps2unix(gps_time);
    std::cout << "TimeUtil::Gps2unix\n\tGPS time: " << GLOG_TIMESTAMP(gps_time) 
              << " -> Unix time: " << GLOG_TIMESTAMP(unix_time) << std::endl;

    // https://github.com/ApolloAuto/apollo/blob/master/modules/drivers/gnss/util/time_conversion.h
    unix_time = gps2unix(gps_time);
    std::cout << "gps2unix\n\tGPS time: " << GLOG_TIMESTAMP(gps_time) 
              << " -> Unix time: " << GLOG_TIMESTAMP(unix_time) << std::endl;

    // https://github.com/ApolloAuto/apollo/blob/master/cyber/common/time_conversion.h
    unix_time = GpsToUnixSeconds(gps_time);
    std::cout << "GpsToUnixSeconds\n\tGPS time: " << GLOG_TIMESTAMP(gps_time) 
              << " -> Unix time: " << GLOG_TIMESTAMP(unix_time) << std::endl;

    return 0;
}

There is a 2 seconds diff, between time_util.h and time_conversion.h, as shown below

(msfloc) bo@apollo:~/projects/time_conversion$ ./a.out 
TimeUtil::Gps2unix
    GPS time: 1001696182.543048024 -> Unix time: 1317660965.543047905
gps2unix
    GPS time: 1001696182.543048024 -> Unix time: 1317660967.543047905
GpsToUnixSeconds
    GPS time: 1001696182.543048024 -> Unix time: 1317660967.543047905

Which one is correct?