If you type a date with a two digit year, Excel treats anything up to 29 as being 2029. Once you get to 30, though, they assume you mean 1930. With 2030 only eight years away, it might be time to adjust this.
Microsoft tries to document how, but the Help topic does not match the Windows 10 Control Panel. This video shows you how to find the settings.
Fixing the 2029 Rule in Excel.