When generating self signed certificate this field sets the valid fields. 'Valid from' is current GMT and 'valid to' current GMT + the value of this field.
Sources for this description
Multiple manufacturers; see the original MIB MIB copies (LibreNMS/Observium Community)Dell MIB copies (LibreNMS/Observium Community)Netdisco MIB archive copiesNETGEAR official product and EOL MIB downloads