I recommend keeping a stable time to write. It helps to psych yourself and schedule that daily writing time. It'll become a habit before you know it and then when you do happen to skip time, you'll feel very unproductive, almost lost. It wasn't my fault that I had to switch things up, it just happened to be that my parents needed me to be home while they were out of state. At first I panicked when I couldn't get in the writing muse. I nearly lost my mind and felt very useless and helpless. I didn't know what to do to wake up those creative juices. I stood, twirled around, drank water, shook my head, took deep and long breaths, listened to my favorite music, and read old posts and manuscripts, as well as pieces I'm currently working on. Nothing helped. Finally I decided to just write, no matter how crappy or senseless it sounded, I was going to do it. I did. And what I wrote was surprisingly good. So what was all that about? I was out of zone. It wasn't my usual time and I'm sure if I would have changed all of the clocks to 3:30 I would have been convinced it was time to write. I pressed on though. I realized that I needed to be flexible with my writing, just as with life in general.
Writing and life go hand in hand. If you cannot be flexible in life and adapt to the changes that swing your way, you will not be able of accomplishing very much when things get messy. Writing is the same. Writing is fun. Writing is a dread. Writing is joyous. Writing stinks. Writing is all of the above and more. I highly recommend setting a routine and writing at that time. I also think it's a great idea to write to a certain amount of words or pages, or time, however you like to count it. I write every day in my current project until I reach 1,000 words. Once I reach that many words I either decide to stop or go on--I always carry on for another 200 or so. It's important to set those goals for your day and reach them. A serious writer will set goals and try to reach them. Set attainable goals, goals that you can see reaching in a day, not a lifetime. I'm always pleased when I reach my word quota for the day.
So what can you gain from all of this? Be flexible. But set a schedule too. It definitely won't hurt you, but be reasonable. If you train your body to write at a certain time of the day it'll do it. I write every morning and every afternoon. I tried writing in the evening last week and I liked it, not as much as the afternoons, but it definitely wasn't a blooper. Try different times and find your time. Perhaps you're one of those writers who can write anytime of the day, if not, that's okay. Honestly, I'm not. I get confused when I'm not in my chair at the same time each day, but if the change happens, accept it and move on. Just write. That's the only help.
Write because you cannot stop writing. Write because you love it. Write because it's part of your life.