No, it doesn't have to be at the same time every day. You can change it to fit your time frame (schedule). Do what works best for you when it comes to meditation as well as what time you're going to meditate. Remember it's not always nor mostly how long you meditate for or when you do so as well as what day you decide to do it on it's about doing it & being dedicated to doing it for your health.
It is always okay to adapt any time to fit you and be okay. Don't stress yourself on timing, it just should be time for you !