I've used a small program that auto increments a number in a batch file to provide a unique id. I tried to find an example of sd_event using this number but I cannot find the batch program.
If you look at this example, you can see how the auto number works in a batch file. Since you only perform the exchange monthly, you probably won't need a lot of unique numbers although you may need to be careful if there are several programs using sd_event through batch.
You might also be able to use the date and time from the machine as a unique variable by calling the date command from inside the batch and storing it in a variable.