Section 3.3.2, NIST SP 800-52:
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The server random value, sent in the ServerHello message, contains a 4-byte timestamp[*] value and 28-byte random value. The validated random number generator shall be used to generate the 28-byte random value of the server random value. The validated random number generator should be used to generate the 4-byte timestamp of the server random value.

Section 4.3.2, NIST SP 800-52:
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The validated random number generator shall be used to generate the 28-byte random value of the client random value. The validated random number generator should be used to generate the 4-byte timestamp of the client random value.

[*] The timestamp value does not need to be correct in TLS. It can be any 4-byte value, unless otherwise restricted by higher-level or application protocols.