Sorry for the confusions but Cornerstone/Content builder has no native Countdown element. There is a Counter element but that is counting up.
Based on the screenshot you provided above, that seems to be the Under Construction plugin, however, I can not replicate the issue on my end.
As you can see I was able to set that to 99 days. Please configure your Under Construction plugin and set the Completed by option, 99 days from the current date.
Extension - Under Construction
Please clarify, Thanks,