Zero pad for empty digits

[GhostToast]

How do you successfully implement zero pad? I have a 3-digit-long day countdown (ddd:hh:mm:ss) but when i experiment and use a shorter destination (dd:hh:mm:ss) I want to see a 0 in the first place, indicating less than 100 days till event - for styling/spacing purposes.

Does anyone know how to achieve this? I tried looking everywhere for this, and I even see one example in test.html that seems to be implementing this successfully (though perhaps with different fields), but no indication of how it's being done. Thanks.