Linked chart date format changes to numbers

[Meaty66]

I have a dashboard in a workbook that displays a number of charts that have their data stored in a different workbook. This is so that we can update the data upon demand and independently of the dashboard (as it may have a number of concurrent viewers). It works pretty well on the whole, but some charts seem to revert their x-axis date formats to numbers rather than a recognisable date format. If the source workbook is open then there is no problem and the dates display as they should, but if it's not, then they change back to numbers. Is there a way I can keep the current structure but maintain the format that I choose?

Thanks in advance

Mark