You are working with a delimited list of dates and they are in the wrong country format, so your Numbers or Excel spreadsheet doesn’t recognise them.
For example, US dates are MM/DD/YYYY whereas UK dates are DD/MM/YYYY
If your Mac is set to UK localisation preferences, a big list of dates from an external source might not be recognised as dates in Numbers or Excel if they are in US format.
Worse still, it might read some of the dates incorrectly. For example 02/05/2016 in US format is February 5th 2016, but your spreadsheet (in UK mode) will detect it incorrectly as the 2nd May 2016.
When it tries to process a US date in UK format which is impossible e.g 02/25/2016, it will treat that as plain text rather than a date. In US format this is a valid representation of the 25th Feb, but in UK format you can’t have the 2nd day of the 25th month, so it is converted to a text cell. Nightmare.
You need Excel for this, tested on 2011+.
Let’s say you want to convert US dates MM/DD/YY to UK DD/MM/YY:
- Paste the dates into a column in a new Excel spreadsheet
- Select the whole column
- Select the Data menu > Text to Columns
- Choose “Delimited” and click next
- Untick all Delimiters and click next
- Select the dropdown menu format “Date: MDY” and click Finish
- The dates will be swapped around to the UK standard and hence will be recognised as such if you then wish to order that column by date