In this case no picture management tool was involved, but along the way something happened.
By chance, another conundrum. I downloaded all my iPhone photos onto an external HD last October to free up space on my phone to take further photos. On looking at them recently, they all have the exif date of 5th October 2019, which must have been the date I transferred then to the external HD. Before this they were all in a normal date/time order. I did nothing knowingly, yet now they're all the same date! Anyone have any ideas ?