The reason why Apple's desktop time becomes transparent may be because your operating system has updated or you have made some personalization changes. Apple's operating system usually provides some options for adjusting the appearance of the desktop, including transparency. You can try to find the desktop or appearance related options in system Settings to see if there is an option to adjust transparency.
If you can't find it, try searching for third-party software or plugins that may offer more desktop personalization options, including transparency adjustments.