Need to generate a unique 5 digit number from 00001 to 99999 in C++

We will modify part of the existing menu logic to generate an ID for the user. This user ID will be of the type int, and will be 5 digits in length. The values are 00001 – 99999, and should appear with all 5 digits (leading zeroes) on the display upon generating them. Do not change the data type from that of an int. Check out the setfill manipulator as a possible helper for this. Setfill allows you to set a fill character to a value using an input parameter, such as ‘0’ for leading zeroes. Researching it, you’ll see that you will need to reset the fill character after use too, or you will be printing a lot of zeroes on the display! Your program also should guarantee an absolutely unique user ID has been generated for each new activity that is added. Think about how this works...

Currently I've been trying to get the following code to work

srand(time(0));
cout << setfill('0') << setw(5) << rand() %99999 << endl;

Problem is that this doesn't seem random at all (it's just slowly counting up based on the computers internal clock right?) and the first digit is always zero. Like the instructions say it should be between 00001 and 99999.