سَافَرْتُ شَهْرًا — shahran is an accusative ẓarf with understood fī.
سَافَرْتُ فِي شَهْرٍ — now fī is expressed; shahrin is genitive.
Similar temporal meaning does not imply the same surface parsing. The second is an added example.
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Implied versus Expressed Fī
Al-Mafʿūl Fīhi: Adverbials of Time and Place
سَافَرْتُ شَهْرًا — shahran is an accusative ẓarf with understood fī.
سَافَرْتُ فِي شَهْرٍ — now fī is expressed; shahrin is genitive.
Similar temporal meaning does not imply the same surface parsing. The second is an added example.