With respect to traditional IRAs, contributions may be deducted for that tax year if the contribution is made on account of that year. This applies both to contributions to individual plans and contributions to spousal plans.1
A postmark is evidence of the timeliness of the contribution.2
1. IRC §§ 219(f)(3), 408A(c)(6).
2. Let. Ruls. 8633080, 8611090, 8536085.