If my tdee is, round figures, 1700 and I stick to, say 1200, I should be able to shift some weight according to my app. So, can I have a day where I go over, then go under to make up for it another day? I thought this was OK? But perhaps I am doing it wrong! 
Last week I was slightly over, but while one side if me says 170 calories for the week is nothing major to stress about, the other side of me is panicking. I had a dinner out on Saturday night, and a celebration at work one day, so did have a light day to help compensate, but only one day for the week was over my tdee.
Or have I got it a terribly confused?
I am new to calorie counting!