Keep getting HTTP 400 errors using ESP8266HTTPClient - firebase

Question

I'm trying to POST data to a URL using the ESP8266HTTPClient library, but I keep getting 400 errors. I verified that I can POST to that URL via Postman, so I'm not sure why it keeps failing with this sketch.
The output is:
Sending data to https://my-project.firebaseio.com/users/MY_USER_ID.json
{"timestamp":"2021-10-28 14:20:00","duration":"1"}
HTTP Response code: 400
Full code below:
#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>
// WiFi configuration
const char *ssid = "MY_SSID";
const char *password = "MY_PASSWORD";
// Database configuration
const String userId = "MY_USER_ID";
const String baseUrl = "https://my-project.firebaseio.com";
void setup()
{
delay(1000);
Serial.begin(9600);
Serial.print("Configuring access point...");
WiFi.begin(ssid, password);
while (WiFi.status() != WL_CONNECTED)
{
Serial.print(".");
delay(500);
}
Serial.println("");
Serial.print("Connected to WiFi network with IP Address: ");
Serial.println(WiFi.localIP());
}
bool shouldSendData = true;
void loop()
{
if (shouldSendData)
{
Serial.println("Attempting to send data...");
if (WiFi.status() == WL_CONNECTED)
{
WiFiClient client;
HTTPClient http;
String url = baseUrl + "/users/" + userId + ".json";
http.begin(client, url);
http.addHeader("Content-Type", "application/json");
String data = "{\"timestamp\":\"2021-10-28 14:20:00\",\"duration\":\"1\"}";
int httpResponseCode = http.POST(data);
Serial.print("Sending data to ");
Serial.println(url);
Serial.println(data);
Serial.print("HTTP Response code: ");
Serial.println(httpResponseCode);
http.end();
}
else
{
Serial.println("Unable to send data. WiFi is not connected.");
}
shouldSendData = false;
}
}