M41T81REG_HR
tm.tm_hour = (tm.tm_hour & 0x3f) | (m41t81_read(M41T81REG_HR) & 0xc0);
m41t81_write(M41T81REG_HR, tm.tm_hour);
hour = m41t81_read(M41T81REG_HR) & 0x3f;