Because data's type is string, when it's key and value's type is also string, then we should enclose it with quotes. like this:
the original json structure: {"date": "2021-10-18"}
after print with cJSON_Print, char * data = "{"date" : "2021-10-18"}"

so if you printf data, the date of 2021-10-18 absolutely wrapped with quotes

I am afraid I dont think that's the case, such the example:
char date[11] = "2021-10-18"; printf(date);
The print result is 2021-10-18 not "2021-10-18"
but i printf("cJSON_Print date") result is "2021-10-18"
so i can not compare them
By the way, I am a java developer

the resulting print 2021-10-18 is still char* not the number. you still not able to compare with numbers directly.
"2021-10-18" just add character \" at the beginning and the ending.

I am afraid I dont think that's the case, such the example: char date[11] = "2021-10-18"; printf(date); The print result is 2021-10-18 not "2021-10-18" but i printf("cJSON_Print date") result is "2021-10-18" so i can not compare them By the way, I am a java developer

What cJSON_Print is always in legal JSON syntax. 2021-10-18 without quotes is not legal JSON.

I am afraid I dont think that's the case, such the example: char date[11] = "2021-10-18"; printf(date); The print result is 2021-10-18 not "2021-10-18"

Yes, this absolutely print 2021-10-18.
But if char date[13] = "\"2021-10-18\"", then print will be "2021-10-18"

but i printf("cJSON_Print date") result is "2021-10-18" so i can not compare them

At first, you can't pass date which type is char* to cJSON_Print, we must pass a cJSON item to it.
in RFC-8259, a valid simple json only contains value:

 Here are three small JSON texts containing only values:
 -  "Hello world!" --> string, must be wrapped with quote: "2021-10-18"
 -  42 --> number
 -  true --> boolean

so let's parse the date:
cJSON* item = cJSON_Parse(date)

• if char* date = "2021-10-18",
  parsing fails, because now we pass a json text like 2021-10-18 to the parser, It is neither a string nor a number.

• if char* date = "2021",
  parsing success, item->type is cJSON_Number
  then cJSON_Print(item) will return a number 2021

• if char* date = "\"2021-10-18\""
  parsing success, item->type is cJSON_String
  then cJSON_Print(item) will return a string "2021-10-18", and using printf print it, it will be "2021-10-18"

By the way, I am a java developer

let's explain the demo with fastjson(version1.2.58, a java json lib)

public void test() {
        String json = "\"2021-10-18\""; // if json = "2021-10-18", then parse will fail
        Object obj = JSON.parse(json);
        System.out.println(obj);

        String str = JSON.toJSONString(obj); // equivalent to cJSON_Print in cJSON
        System.out.println(str); // "2021-10-18", the same with cJSON
}