Yī jiǔ sì jiǔ nián shì xīn Zhōngguó kāiguó de niánfèn.
1949 was the year the new China was founded.
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usage
开国 most often appears attributively in compounds: 开国皇帝 (founding emperor), 开国元勋 (founding hero/father), 开国大典 (founding ceremony — specifically the Oct 1, 1949 ceremony in Tian'anmen Square that proclaimed the PRC).