– Delta Air Lines is hoping to make waiting on your bags a little less annoying with a new on-time delivery guarantee. The catch: It’s not permanent.

Under the promotion’s terms, the Atlanta-based carrier promises to have your luggage on the carousel in 20 minutes or less. If it fails to do so, it will credit your frequent flier account with 2,500 bonus miles if you’re a SkyMiles member. The guarantee holds for domestic flights nationwide through March 31.

Delta is copying Alaska Airlines, which introduced a similar guarantee in 2009, said Jay Sorensen, an aviation consultant with IdeaWorksCompany in Shorewood, Wis. What’s odd about Delta’s move is that it’s temporary, while Alaska offers a permanent 20-minute promise, he said.

“I give them negative points for this guarantee,” Sorensen said. “The fact they have placed a date limit of March 31 on good service says they’re not treating this seriously.”

Anthony Black, a Delta spokesman, said in an e-mail that the bag guarantee “is a test.” He didn’t respond to a question asking whether Delta is considering a permanent guarantee. The company’s website doesn’t say the offer is a trial or it may be extended.

Delta has been battling Alaska for market share as it builds out its new Seattle hub, which it sees as a gateway to the Pacific region. The carriers have showered Seattle-based customers with perks in an effort to win their business, including Delta’s offer last year of double SkyMiles on flights from Seattle to certain West Coast cities.