Kenko KK628 Calculator watch - Devon Bostick - Dead Before Dawn
On the film poster the watch can clearly be seen on the wrist of Devon Bostick and in the first part of the film the watch is worn by character Casper Galloway (played by Devon Bostick).
The KK628 watch in the film is a grey plastic watch with calculator. Made in China and sometimes branded Kenko KK-628, but also sold without the Kenko logo. Kenko is a manufacturer of calculators, visit their website here. The watch in the film seems to have only the text KK628 on the center top of the screen, and not the Kenko logo. The watch is also available in many other colors including black, red or yellow. Watch a video about the watch here.
Because it doesn't really have a brand name, the watch is hard to find online. On eBay you can find it if you search for "calculator watch". You can get the grey KK628 watch online in the UK or Europe via Amazon.co.uk for £5.99.