Unlike today's troop deployment in Iraq where the soldiers have no clue when they will come home, in Vietnam you knew! You had one year, 365 days, sometimes less, NEVER more. Hence, tracking how many days you have before you leave was a part of everyday life. As important as eating. Thus grew the concept of the "Short timer". As the number of days you had left got smaller, those with more days looked upon you with envy. And those that were truly "short" were proud of it. Everyone lived for the day they became a "single digit midgit".
Here is the calendar on which I personally marked off each day. I made it the first week in country. It was posted in our building. Soon others started using it to track their own days. They added the comments you read. It is a visual reminder of those days spent one day at a time. I was lucky, I came home 18 days "short" of 365.
So there you have it.