If you're age 16 or older and your employment contract states that the employer can make you work 12 hours or more per day then yes the employer can legally make you work over 12 hours per day if you want to keep your job.
Many people do work up to 14 hours per day and I would gladly work 14 hours per day especially if it was in an office and the weather was brutally cold or was too bad to be outside anyway.
If you don't like working 12 hours or more per day then you should look for another job that only works you for 5 to 8 hours per day.