Directors of Time Inc. and Inland Container Corp. yesterday approved an agreement to merge the sizeable corrugated container manufacturer into the diversified communications company.

The proposed acquisition is worth about $280 million and will be accomplished through an exchange of Time Inc. securities for up to two-thirds of Inland Containers' 8.03 million shares, and a cash offer $35 a share for the rest.

Inland Container, based in Indianapolis, earned $21.9 million on sales of $396.7 million in 1977. It manufacturers corrugated shipping containers and container board at 28 plants in the United States and Puerto Rico.

Inland also has half ownership with Mead paper Co. in Georgia Kraft Co. which produced 1.2 million tons of linerboard last year, and which owns 950,000 acres of timberland in the southeast U.S., and leases another 125,000 acres.

Time, while best known for its magazine and book publications, last year derived more than a quarter of its sales and nearly one-third of its earnings from Temple-Eastex, its largest subsidiary, which turns timber into paper products and home building materials.

In 1977, Time earned $90.5 million on revenues of $1.25 billion.

Should the merger go through, a company spokesman estimated that, based on last year's results, Time would derive 51 percent of its earnings from publishing. Forty-five percent would come from forest products and the remainder from television and other miscellaneous operations.

The agreement in principle to merge is still subject to the negotiation and execution of a definitive agreement, according to the announcement, as well as approval from the Securities and Exchange Commission and the receipt of a favorable tax ruling on aspects of the transaction.

Inland Container would become a wholly owned subsidiary of Time Inc. when the merger is completed, and will be operated by its existing management.