You are correct in using the replace() function, you just have to nest it in a text field. The syntax to use is: replace(yourDateField; “-”; “”)
This removes every dash from the string. If Zoho chokes on the space or +0000 offset, chain replace() calls or run it through formatDate() to control the output pattern to the format you need.
Better to try the formatDate option as it is more reliable than replacing the string, it lets you match Zoho’s expected format better. I use it all the time.