When I was about 29 years old I got a spiritual  “assignment” on an Easter morning in the old city of Jerusalem. According to this assignment my job was to live as if I had only 6 months left to live. I was in perfect health and in the middle of a ten-year round the world trip, so this interruption was unexpected and strange. I’ve told the full story of that curious mission on the very first episode of This American Life, the public radio storytelling hit, 10 years ago, so I won’t go into further detail because you can hear my account  on this streaming audio file from the NPR site.

(An almost biblical scene I encountered in my travels.)

The short version is that what I decided to do in my last 6 months surprised me, and that living with only 180 days in front of me turned out to be harder than I thought. But I did live with a very conscious countdown toward the final day; I remember that last day very well.

I am now 55 years old. Like a lot of people in middle age my late-night thoughts bend to contemplations about how short my remaining time is. Even with increasing  longevity there is not enough time to do all that I want. Nowhere close. My friend Stewart Brand, who is now 69, has been arranging his life in blocks of 5 years. Five years is what he says any project worth doing will take. From moment of inception to the last good-riddance, a book, a campaign, a new job, a start-up will take 5 years to play through. So, he asks himself, how many 5 years do I have left? He can count them on one hand even if he is lucky. So this clarifies his choices. If he has less than 5 big things he can do, what will they be?

Another friend, a musician, told me about a recurring dream he had in which he could see the exact number of days left in his life. His days were numbered, literally. He recounted how invigorating this knowledge was, because while he could never be certain that number was true, it did help him prioritize his choices and defuse his procrastinations.

I decided to take the idea of number days seriously, and to revisit my earlier experience of counting down my remaining time on this lovely mortal plane. My hope was that a reckoning of my numbered days would help me account for how I spend each precious 24 hours, and to focus my attention and energy on those few tasks and projects I deem most important to me. Indeed, it might help me decide which ones are most important, which is the harder assignment.

What I wanted was a great big flashing sign that would show up on my computer and shout out to me how many days I had remaining. Then I would try to use my blog to record what how I spent the day to keep me honest. A wasted day would reveal its loss in the empty lower count the next day. I figured that mounting an automatic personal countdown sign should be pretty easy, and something that others might want to do as well.

My first step was to figure out my expected longevity. There are two ways to do this. One is to take the general average life-span for a guy my age in the US. To get that figure all you need to do is go the standard Life Expectancy Table (PDF), like the ones published by Medicaid. This will tell you the age at which statisticians expect you will die. For me, its 78.68 years.

The other way is to amend that basic figure with the plus and minus of your personal lifestyle, known illnesses and family health history. There are various websites that purport to make that calculation but I don’t believe them. I believe they could make this adjustments with sufficient data, but I don’t believe these websites have the proper scientific judgement or data to do so. So I’ll stick the less exact but more trustworthy average age.

Either way you go, you take your expected life span in years and starting from your birthday figure out how many years plus days you have left. Then you go to the Date Duration Calculator to transform the remaining time into a specific date. For me the estimated last day of my life turned out to be easily remember. I am suppose to die on New Years Day (January 1), 2031. I sure hope I have a really really good time the night before.

To turn that end date into a current “days left” you can use one or more free countdown clocks. There is a Google gadget that sits on your iGoogle homepage, called not surprisingly Countdown Clock. You enter your last day and from thence on it will show you how many days until then.

I also wanted a desktop clock for when I was not online. I am on a Mac and there is a tiny app called Days Left that displays the remaining countdown days in the dock. For Windows there is TimeLeft which I have not used. If you want to roll your own web-based countdown clock, there are some explicit instructions here for programming a customized one.

I’ve been using this system for several months now and it has been very powerful. Day to day I  am aware — and can rattle off if I am asked – how many days I have left. I decided to post my project today because on my clock it shows a handily rounded off sum. So here is the news: As of today I have 8,500 days left to live.  That’s not much in my book. I can almost hear them ticking away as we speak. I look at my lifelist of current dreams and I realize that in only 8,500 days I won’t get to but a few of them. And what of any new dreams?

Of course I could die next month and not get to any of them. Or I could live to be 99 and have an extra 5,000 days.  Man, in that case I will be so happy to have to reset my clock! For now, the countdown clock, even armed with an average date, helps me focus. I don’t think having the actual date of my death would change much. The time left is still too short. And too close. And getting closer. And I’m sorry but I need to do something else right now….