Date problem - date showing differently in different spreadsheets???

[jemgilks]

I have data in a spreadsheet which I have transferred to another spreadsheet. Both Excel spreadsheets.

In source spreadsheet the date shows as 21/05/2019 and serial number is 42144
In my new spreadsheet that serial number shows the date as 20/5/2015

try as I might I cannot work out why this is. I feel it must me a different calendar applied somewhere but google searches are not helping.
Any help much appreciated

 

[Fluff]

One workbook is using the default date system & the other is using the 1904 date system.