Family visit

Hi there, im in london for 3 years under student visa. I gota go to china for my sister's convacation in july. Im flying london-colombo and after 2 weeks colombo -china. When i tried to apply in chinese embessy in London, i've been told i will get visa only for a month from the date of my application. As i am staying in colombo for 2 weeks time, i don't think i would be able to spend my time in china as i planned. I would like to know if i can request a date from when i need the visa to be redemed?
Regards