What is the burden of proving that a delay in disclosing expert opinions was harmless?

See Quevedo v. Trans-Pac. Shipping, Inc., 143 F.3d 1255, 1258 (9th Cir. 1998) (refusing to consider expert's report because it was filed one-and-a-half months late and plaintiff could have asked for an extension of time).

Nor has Deckers shown that the delay was harmless. Deckers asserts that the burden of proving harm is on the party seeking sanctions; we disagree. Implicit in Rule 37(c)(1) is that the burden is on the party facing sanctions to prove harmlessness. At least one other circuit court has so held. Wilson v. Bradlees of New England, Inc., 250 F.3d 10, 21 (1st Cir. 2001) ("[I]t is the obligation of the party facing sanctions for belated disclosure to show that its failure to comply with [Rule 26] was either justified or harmless . .. ." ).