Which timer library to use for several seconds of delay

Not sure, but maybe this will help you?

unsigned long elapsed;
bool pinstate;

void InvertState(unsigned long duration)
{
  if (millis() - elapsed >= duration)
  {
    pinstate = !pinstate;
    elapsed = millis();
  }

  print(pinstate ? ("I'm HIGH") : ("I'm LOW"));
}

void loop()
{
  InvertState(500);
}

Edit: Thanks Coding Badly and PaulS, I have some difficulties with all those data types..