Hi Younes. Great to hear from you again! So, Dr. Sacks is a pretty amazing researcher who continued to use himself and his condition for research. He knows how fast he writes is a literal statement. He is aware from experience how many minutes it takes him to write (by hand) say one page of information. He was actually conscious of the fact that he was entering a physical state of delirium. So he continued writing so that when he came out of the state of delirium he could determine the length of time he was "out of it" based on the amount of writing on the page or pages. That is how he would develop the "timeline". He didn't really get to finish this explanation clearly because the interviewer interrupted once he understood what Dr. Sacks was explaining because he (the interviewer) was so amazed by the event described. Delirium is defined as "a serious disturbance in mental abilities that results in confused thinking and reduced awareness of the environment." So one must assume that Dr. Sacks could sense the onset of an episode of delirium and wanted to gauge the length through his writing measurement method ... and of course what he would have written during that time as long as the delirium didn't cause him to stop writing. Hope this helps! Have a lovely day, Ellen