The RD is the ND on the receipt.
Here is my understanding:
We have RD and ND on the receipt. The RD is the date INS received our application. The ND is the date they notify us (the date of dispatch of receipt.)
Now, the ND on our receipt becomes RD for INS actions. They issue SRC number based on the RD (the ND on the receipt). If you see the data in the data consolidate project, this becomes clear.
Forget about the RD on receipt date. The ND on the receipt becomes RD. So counting starts from ND.