One of the most serious issues dealt with in Christopher Nolan's Memento, is the grasp of time -- the hero seems to be "lost in time". Throughout the entire picture the audience, and the hero himself, struggle to understand how much time has passed since the accident, which caused his condition. Which basically means - how much time has passed since he started this intensive "quest".
When the movie just starts, the audience tends to think that this whole situation is considerably "fresh"; but as the plot progresses (or actually, goes backwards), one might consider the un-optimistic fact that this "quest" might be taking place for years, and maybe the real goal was reached a long time ago, but for some reason (maybe a third party's interest, or a selfish decision) the hero is trapped in a "loop". I mean, he can't really trust his notes, Polaroid photographs and tattoos for this matter, because they all could be theoretically "sabotaged" to serve someone's interest. In the ending scenes it is even implied that it might have been the hero himself, who "sabotaged" his path in order to continue this quest; due to the fact that without this "noble" cause, his scarred existence will be empty of purpose.
This situation is very chaotic indeed, and very hard to grasp - but, when thinking about this situation - the most logical thing to do is to obsessively record dates.
I mean, he works with all those kinds of police paperwork, regarding his wife's murder, and all of those papers should have dates on them (and also the tattoos), and If they don't have dates on them by default - it's really easy to find them. It's very easy to find out exactly how much time he's on this quest and it's very easy to know the exact date and time at any given moment (a hand watch with a date function, for example).
How come this easy solution is not considered and the hero just "floats", lost in time?