Convert column of apparent dates to actual date values

I have a sheet with some 2,000 odd records, with a couple of columns of dates

For some reason, the dates are not being recognised as such, despite setting the call formats to 'Date'. It's messing up the formula arrays I have to count records that meet certain parameters!

I've discovered that if I enter a cell containing on of these dates as if to edit it and push Enter, it alters said data and it's suddenly okay... but I really don't have the patience, fortitude, time or wrist flexibility to work through 4,000 dates!

A sample is attached; green have been 'fixed', while orange are originals. The only way I can tell the difference between the two is to run a 'Clean' to identify which ones are suddenly dates in Excel's eyes. Is there any way to apply this 'fix' wholesale, rather than going into each individual cell?!

Thanks in advance people!