Some of the best books on writing and the writing lifestyle suggest that you make time every day to write. They suggest everything from an hour a day minimum to all day once a week, to going by number of pages per day.
While those books have the write, I mean right, idea, sometimes they can be a little unrealistic for those with time constraints to follow. It doesn't allow for emergencies, unexpected company (sometimes that is an emergency!), acts of God, etc.
I agree that it's important to make time to write. However, if you cannot make time every single day, you shouldn't say to yourself, "Oh, I'm not following the suggestion in the book, therefore I'm not a writer." Nonsense. You're a writer if you love to write and do it even occasionally. You're an author if you persevere to the point of publication, which means setting ego aside and taking constructive criticism about your work, making edits, and trying to fit into the buying market.
And, if you can't write more than one sentence a day, make that sentence count!