expiry

/* guard_limit must be larger than guard_secs */
#define EXPIRY_GUARD_SECS  15
#define EXPIRY_GUARD_MIN   3
#define EXPIRY_GUARD_LIMIT 30
#define EXPIRY_GUARD_PCT   0.2

if(expires <= EXPIRY_GUARD_MIN)
  expires = 1;
else
  if(expires < EXPIRY_GUARD_LIMIT)
    expires -= MAX( (expires * EXPIRY_GUARD_PCT), 2 );
  else
    expires -= EXPIRY_GUARD_SECS;