Try to get a correspondence from them stating your regular working hours. Even better if your employment letter mentions the working hours. In addition to those you may get reference letters from co-workers and senior employees stating that 'full time' implies so and so hours w.r.t to the policies of the company.
Also, look for other out of the box solutions such as working hours mentioned on any card/letter pad/website of the company.
All these (as much as you can get) can then be compiled together along with a letter of explanation stating that you weren't able to get an official letter stating your working hours.