When Rust Expects a String But Gets a Map

I recently hit this error while working with a Rust application that reads from a database:

Database("Kind: invalid type: map, expected an RFC 3339 formatted date and time string, labels: {}")

At first glance, it's a bit cryptic. But the error message actually tells you exactly what's wrong.

The Problem

When you have a Rust struct with a DateTime<Utc> field, Serde expects the JSON to contain an RFC 3339 formatted string like:

"2025-09-15T01:36:19Z"

But instead, your database (or JSON source) is storing it as a map/object:

{
  "sec": 1694733379,
  "nsec": 199610000
}

Serde can't automatically convert a map into a DateTime—it needs a string it can parse.

Why This Happens

Different systems store timestamps differently:

• Some databases store timestamps as Unix time objects with separate seconds and nanoseconds
• Some JSON APIs return timestamps as nested objects
• Other sources might use different date formats

But Rust's DateTime<Utc> with Serde defaults expects the standard RFC 3339 string format.

The Fix

You have two options:

Option 1: Fix the data source (if you control it) Make sure your database or JSON source stores timestamps as RFC 3339 strings:

"created_at": "2025-09-15T01:36:19Z"

Option 2: Use a custom deserializer (if you can't change the source) Write a custom Serde deserializer to handle the map format:

use chrono::{DateTime, Utc};
use serde::{Deserialize, Deserializer};

fn from_timestamp_map<'de, D>(deserializer: D) -> Result<DateTime<Utc>, D::Error>
where
    D: Deserializer<'de>,
{
    #[derive(Deserialize)]
    struct Timestamp {
        sec: i64,
        nsec: u32,
    }
    
    let ts = Timestamp::deserialize(deserializer)?;
    DateTime::from_timestamp(ts.sec, ts.nsec)
        .ok_or_else(|| serde::de::Error::custom("Invalid timestamp"))
}

#[derive(Deserialize)]
struct ModelSelection {
    #[serde(deserialize_with = "from_timestamp_map")]
    created_at: DateTime<Utc>,
}

The custom deserializer reads the sec and nsec fields from the map and constructs a DateTime<Utc> from them.

Bottom Line

When Serde complains about expecting a string but getting a map for a DateTime field, your data source is storing timestamps in a format Serde doesn't recognize. Either standardize on RFC 3339 strings, or write a deserializer to handle your specific format.