Does GS have to be given exactly 24 hours a day? What if I delay the next dose by 3 to 4 hours, or my veterinarian has shortened office hours, can I give the injection earlier? If I do, what will be the best time for the next try the next day?

Occasional fluctuations during administration do not matter as the effective blood level is more than 24 hours. There is some flexibility in dosing, but the constant change in administration time depending on the owner's schedule does not say well about the owner's attitude towards treatment and his cat.