That’s a good question. According to WD, your replacement drive should have shipped with a 90-day warranty, but once they received your bad drive, the warranty on the replacement should have been extended out to match the original:
Why is the warranty on the replacement drive or product shorter than the warranty on my original drive?
"In an Advance RMA, the replacement drives are shipped with a 90-day warranty.
If the remaining warranty on the defective drive is more than 90 days, the remaining warranty will be applied to the replacement drive after we have received the defective product. For example, if the defective drive’s warranty expires on February 21, 2011 and the replacement drive was shipped on December 1, 2009, the current warranty on the replacement drive will be 90 days from December 1, 2009 until we receive the defective product. Once we received the defective drive, the warranty on the replacement drive is updated to February 21, 2011.
If the remaining warranty of the defective drive is less than 90 days, the warranty on the replacement drive is 90 days from the date the drive was shipped."
The warranty on my WD product is wrong. How can I update, correct, or extend the warranty on my WD product?
"If the warranty on your WD product is incorrect, you can have it corrected by registering the drive through our Registration Service. Once it’s registered, the warranty will be updated. In addition, you will know if the drive is eligible for warranty extension.
If your registered drive still show an incorrect warranty date, please send us an email and attach a copy of your proof of purchase. We need your proof of purchase in order to update the warranty to start from the date of purchase."