Using regex or excel, how to convert a date into a decimal?

I have an excel file with 200k rows. In one column, approx. 20% of the values show incorrect data. The problem: the values in the excel should be depicted, for instance, as '1.00' or '2.75'. The faulty rows show data as dates. For instance, '1.1.2026' [Day-Month-Year] instead of the correct '1.26', or '1.5.2025' as '5.25', or '1.2.1975' as '2.75'.

Question: how can I use Excel or Regular expression to transform all my faulty rows to being correct? I have already tried changing the data format to 'Text' or others. Doesn't help (instead, changing the data format to General for '1.2.1975' gives the value '27426', which is incorrect).

Thanks.