Phone numbers and postal codes converted into nonsensical date format

Phone numbers and postal codes converted into nonsensical date format

In the last hour or so, whenever I import leads using spreadsheet, Zoho appears to automatically convert phone numbers and postal codes into nonsensical date format.

I tried importing another spreadsheet which had been imported to Zoho successfully before and I encountered the same issue which indicates that it's not a problem with the new spreadsheet I'm trying to import.

The fields are not custom fields so it couldn't have been caused by something I set incorrectly inside Zoho.

I've attached a screenshot.

Any ideas?