Manually collecting dates is a hassle, and often a waste of time.
Automatically collecting dates - eg if your computer capture tool does that for you - helps a lot. I probably would not capture dates if my tool did not do so automatically.
Back when I did most of my capture by hand in paper “Inventor’s Notebooks” I diligently added the date, and often the time. Note “Inventor’s Notebook”. In the pre-Berne “first to invent” it was important to be able to show a chain of dates if you wanted to prove priority. Less important now. This applies even to tracking tasks - even if you just looked at a task in weekly review and EE-deferred it, that could be useful in proving “diligence in reducing to practice”. Even back in the old first to invent days, if you invented something 10 years before everyone else, but then left it in your someday folder, you lose your rights. Whereas if you could prove that you reviewed it periodically, you might maintain them.
Accountants and workers on government contracts often need to track time for billing. It can be convenient to have date and time tracked ... not necessarily in your GTD system, but when you fall into the habit...
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Most of these things don’t apply so much to GTD for GTD’s sake.
I mainly use dates because my preferred capture mechanism is to use a date ordered log. And then copy or link into GTD folder/lists. Sometimes helps me find some things that I can’t think of the name for.
In fact, I was about to post complaining that various of the capture tools I use - eg iOS Reminders, the Drafts app, or Just Press Record - often Lise day information when I transfer info to OneNote. Making my logs messy.
Shorter scale, I accidentally deleted a month’s with of GTD items. If they had not been dated I would have had a lot of pain recovering them, separating them from the stuff that I did not want to delete.
I confess that I often fall behind in my regular reviews - weekly or monthly. Sometimes I sort by date, and move everything older than a threshold into a Someday list. With a low priority project to sort that list,
Sure, if you stay on top of things you don’t need to do this. But I’m not perfect. And I can’t afford to restart GTD from scratch every time I fall behind. I find that knowing that I am on top of everything less than N months old helps ease my mind, even if there is older stuff that I am not so sure about. But I am still sure that I can find the older stuff if I need it.