Horizontal Axis Labels - impossible to change date

I have to fill in a weekly spreadsheet for my boss, that she has created.  She has created a chart to go with this, that usually works OK when I update it, but it has recently gone haywire.  (I am not allowed to touch the design.)

The problem is the horizontal axis labels, which are behaving in a very stupid way.  The labels look like this: "='*********'!$L$3:$L$152" in the chart, and what I do is to change that last figure when I add lines in the spreadsheet.  What I also do regularly is to go to the Axis Options and change the date in the Maximum Bounds box - I usually do this every month or so, but due to some personal changes with my boss, this has not been updated for six months.  Here is how those options are set up:

On this occasion I have been asked to change the Maximum Bounds date to a year on, i.e. 28/12/2023.  But every time I do this now, the date changes to 28/12/2027, which is most certainly NOT what I have put in!  (This of course destroys the chart, so my only option is to undo it.  If I keep fiddling with it, the dates change in all sorts of weird ways, including creating some in 1904!)  It really is the most ridiculous thing, and I can't understand why it's stopped working?

I wonder if anyone could kindly tell me what on earth is going on here?  Surely I should able to override anything in these boxes, without Excel butting in and messing it up?  (Or worse - is my Excel broken?)

I would really appreciate some help here - thanks in advance!