Since you don't provide specifics (like make, model, Android version), I can only provide suggestions of a general nature. You can add it back by either long-pressing a blank spot on the home screen and then selecting Widgets, or access your app drawer and there may be a tab for widgets. Either way, scroll to find the missing widget, then long-press it. It will take you to the home screen you started on, where you can slide it around for placement (provided you have room).
If the time/date widget is not in the scrolling list, then it is either corrupted, deleted, or (if it was a pre-installed system app) disabled. GL!