Can I buy the train tickets two weeks in advance?

Hi! I will travel to China the 14th of September. I want to go from Beijing to Shanghai the 18th of September and back to Beijing the 20th of September. I've booked the hotel in Beijing and I'll be interested in picking up the train tickets there. My question is: Is it possible to buy the tickets 12-14 days in advance? I've read in other webs that you can only buy a ticket ten days in advance for bullet trains. Is it true?

Thanks for your help!