Datetime parsing with strptime is apparently not portable.

With the following code:

#include <ctime>
#include <string>
#include <iostream>

std::time_t parse_datetime(const char* format, const std::string& stamp)
{
  struct tm tm;
  auto res = ::strptime(stamp.data(), format, &tm);
  if (!res)
    return -1;
  return ::timegm(&tm);
}

auto test(const std::string& stamp)
{
  const char* big_z = "%FT%T%Z";
  const char* small_z = "%FT%T%z";

  std::cout << "[" << stamp << "]" << " with [" << big_z << "]";
  std::cout << " --> " << parse_datetime(big_z, stamp) << std::endl;

  std::cout << "[" << stamp << "]" << " with [" << small_z << "]";
  std::cout << " --> " << parse_datetime(small_z, stamp) << std::endl;

}


int main()
{
  test("1970-01-01T00:00:00z");
  test("1970-01-01T00:00:00Z");
  test("1970-01-01T00:00:00UTC");
  test("1970-01-01T00:00:00");
}

OpenBSD 6.0,:

[1970-01-01T00:00:00z] with [%FT%T%Z] --> -1
[1970-01-01T00:00:00z] with [%FT%T%z] --> -1
[1970-01-01T00:00:00Z] with [%FT%T%Z] --> -1 
[1970-01-01T00:00:00Z] with [%FT%T%z] --> 0  
[1970-01-01T00:00:00UTC] with [%FT%T%Z] --> 0
[1970-01-01T00:00:00UTC] with [%FT%T%z] --> 0
[1970-01-01T00:00:00] with [%FT%T%Z] --> -1
[1970-01-01T00:00:00] with [%FT%T%z] --> -1 

Fedora 24

[1970-01-01T00:00:00z] with [%FT%T%Z] --> 0
[1970-01-01T00:00:00z] with [%FT%T%z] --> -1
[1970-01-01T00:00:00Z] with [%FT%T%Z] --> 0
[1970-01-01T00:00:00Z] with [%FT%T%z] --> 0
[1970-01-01T00:00:00UTC] with [%FT%T%Z] --> 0
[1970-01-01T00:00:00UTC] with [%FT%T%z] --> -1
[1970-01-01T00:00:00] with [%FT%T%Z] --> 0
[1970-01-01T00:00:00] with [%FT%T%z] --> -1

Debian

[1970-01-01T00:00:00z] with [%FT%T%Z] --> 0   
[1970-01-01T00:00:00z] with [%FT%T%z] --> -1  
[1970-01-01T00:00:00Z] with [%FT%T%Z] --> 0   
[1970-01-01T00:00:00Z] with [%FT%T%z] --> -1  
[1970-01-01T00:00:00UTC] with [%FT%T%Z] --> 0 
[1970-01-01T00:00:00UTC] with [%FT%T%z] --> -1
[1970-01-01T00:00:00] with [%FT%T%Z] --> 0    
[1970-01-01T00:00:00] with [%FT%T%z] --> -1   

FreeBSD 11.0

[1970-01-01T00:00:00z] with [%FT%T%Z] --> 0     
[1970-01-01T00:00:00z] with [%FT%T%z] --> -1    
[1970-01-01T00:00:00Z] with [%FT%T%Z] --> -1    
[1970-01-01T00:00:00Z] with [%FT%T%z] --> -1    
[1970-01-01T00:00:00UTC] with [%FT%T%Z] --> 3600
[1970-01-01T00:00:00UTC] with [%FT%T%z] --> -1  
[1970-01-01T00:00:00] with [%FT%T%Z] --> 0      
[1970-01-01T00:00:00] with [%FT%T%z] --> -1