سَافَرْتُ يَوْمَ السَّبْتِ — I travelled on Saturday.
- The action is travelling.
- يَوْمَ السَّبْتِ answers when it happened.
- It is an adverb of time.
صُمْتُ شَهْرًا — I fasted for a month.
- شَهْرًا gives the duration of fasting.
- It is also an adverb of time.
A time expression may identify a particular time or a duration.