As @avtraino and @Airy have said, the ICUDateTimeFor token needs the date in unixtime.
In your particular case, if the copied dates are always in the format 30/06/1949 then it is possible to break that string up into its separate parts of Day, Month, Year. Then convert that to unixtime and then apply ICU format to it...
The reason the date needs to be broken into Day, Month, Year is that the TIME() function needs the date in the form of 1949,06,30. It won't work with 30/06/1949.
Here is an example where the Magenta Action sets a Variable to 30/06/1949 (but in your case you would set this from the Clipboard by enabling the second Magenta Action instead).