Is it permissible not to adhere to official work timings with the knowledge of the department head during periods when there are no work tasks? And what is the ruling on signing the attendance and departure register with a time different from the actual time?

If a Muslim agrees to work at a specific time, it is not permissible for him to violate it, due to the Prophet's (PBUH) saying: "Muslims are bound by their conditions," and Allah's (SWT) saying: "O you who have believed, fulfill [your] contracts." And for a worker to sign the attendance and departure log contrary to the actual time is deception. The salary for the hours in which he lied is not lawful for him, and he must compensate the employer or seek their forgiveness.