كَمْ شَهْرٍ صَوْمِي!
كَمْ: declarative, a fronted duration predicate in the nominative position.
شَهْرٍ: genitive declarative tamyīz.
صَوْمِي: delayed topic; its nominative vowel is estimated because of the first-person yāʾ, which is a genitive muḍāf ilayh.
The preceding example used accusative yawman; this one uses genitive shahrin. Both retain predicative kam.