It looks like DateTime_Today is a DateTime type variable in your screenshot, which has a very useful ToString method see documentation on it here: https://learn.microsoft.com/en-us/dotnet/api/system.datetime.tostring?view=netframework-4.7.2 which will allow you to extract the number that you are looking for.
You would need to create a string variable and assign the value to DateTime_Today.ToString("dd") to get only the day of the month.
The issue with this approach, however, is then creating the image for the OCR to look for in the calendar. I think a more reliable way of approaching this issue would be to determine what date is selected by default when clicked and then sending some combination of hotkeys such as tab and the directional keys to get to the date which is required for the automation. A get text or get attribute activity might also help determine what date is currently selected, so you would then know how many boxes will need to be traversed to get the required selection.
If you are still having issues please provide a link to the page which you are trying to automate as well as some (sanitized) code snippets containing what you have already tried and we will be able to assist you further. Cheers!