@Override
  public long nowMillis() {
    return mSystemWallClock.nowMillis() + getTimeCorrectionMinutes() * Utilities.MINUTE_IN_MILLIS;
  }
Cached value of time correction (in minutes) or {@code null} if not cached. The value is cached
because it's read very frequently (once every 100ms) and is modified very infrequently.

@GuardedBy {@link #mLock}
java/com/google/android/apps/authenticator/otp/TotpClock.java