The First World War began 100 years ago, on 28 July 2014, with Austria–Hungary declaring war on Serbia, a month after the assassination of Archduke Franz Ferdinand of Austria and his wife by a Serbian nationalist in Sarajevo. I have always been fascinated by the era and at school studied the gut-wrenching poetry that came out of the trenches. I was also a little bit obsessed by the mathematics of time and by working out whether it was possible for the old soldiers to be alive