The I-751 receipt notice which provides for extension should be for valid for one year from date of expiration of conditional green card unless the notice explicitly states otherwise. Traveling for a few weeks is no problem. Carry the conditional resident card along with the receipt notice extending the conditional resident status to facilitate entry to the U.S.
You are probably looking at the wrong date - look at the dated for which your extension is being approved - should be one year from the expiration of your conditional green card. You should be fine.
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