Pop up alert

Hello,

I have created a worksheet with macro to enter current time by clicking on a cell. For example, double click on A1 to enter current time then double click B1 to enter current time after processing a claim. The total minutes spent on that process will show in C1.

A1 B1 C1
0:09:00 0:09:30 0:00:30

I was just wondering if there is any code in excel that we can use so that after we enter the current time in A, excel will automatically calculate the minutes spent while working on a claim and there will be an alert if it has been more than 20 mins since it has started.

I have only set up conditional formatting in C that the cell will be in red once B is populated for the end time. Or is there anything similar to this effect.

thanks!