Another vote for aging. Add to that the inability, in my case at least, to isolate a particular year from the ones around it because of some personal event that stands out from the rest. That aspect of dating things (as far as years go) began to fade in my 50’s. Relevant stuff had already happened before then and it was just a matter of repetitions after that.
Days or months? Same basic story, except that I used to have memories associated with “warm weather” or “cold weather” because my memories of events usually associate with what sort of clothes I would have been wearing.
My brother and I – whenever we’re in each other’s company long enough to start reminiscing – will try to sequence the “big moments” of our shared lives and will invariably have to rely on trivial associations with memories to decide which event preceded some other one. Sadly, we haven’t been in each other’s presence in so long that even that technique has gotten rusty.