What is the function of "Calculate Now" in Microsoft Excel and what is the shortcut key of "Calculate Now"?

Its helpful in adding date and time.

Current date   Select a cell and press CTRL+;

Current time   Select a cell and press CTRL+SHIFT+;

Current date and time   Select a cell and press CTRL+; then SPACE then CTRL+SHIFT+;

Its manually recalculate all open worksheets, including data tables, and update all open chart sheets, on the Formulas Tab, in the Calculation group,  click the Calculate Now button. (Ctrl+shift+Alt+F9). :)