Track Last Login Time and Update User ACF Field

acf-export-user-field.json

[
    {
        "key": "group_685c37b820182",
        "title": "User Login Info",
        "fields": [
            {
                "key": "field_685c37bd7d63f",
                "label": "Last Login Time",
                "name": "last_login_time",
                "aria-label": "",
                "type": "date_time_picker",
                "instructions": "",
                "required": 0,
                "conditional_logic": 0,
                "wrapper": {
                    "width": "",
                    "class": "",
                    "id": ""
                },
                "display_format": "m\/d\/Y g:i a",
                "return_format": "m\/d\/Y g:i a",
                "first_day": 1,
                "allow_in_bindings": 0
            }
        ],
        "location": [
            [
                {
                    "param": "user_form",
                    "operator": "==",
                    "value": "all"
                }
            ]
        ],
        "menu_order": 0,
        "position": "normal",
        "style": "default",
        "label_placement": "top",
        "instruction_placement": "label",
        "hide_on_screen": "",
        "active": true,
        "description": "",
        "show_in_rest": 0
    }
]

functions.php

add_action('wp_login', 'track_user_last_login', 10, 2);
function track_user_last_login($user_login, $user) {
    if (!function_exists('update_field')) {
        return; // ACF not active
    }

    $user_id = $user->ID;
    $current_time = current_time('Y-m-d H:i:s');

    // Update ACF field (saves to usermeta)
    update_field('last_login_time', $current_time, 'user_' . $user_id);
}

https://developer.wordpress.org/reference/hooks/wp_login/
https://developer.wordpress.org/reference/functions/wp_signon/
https://developer.wordpress.org/reference/hooks/wp_authenticate/
https://developer.wordpress.org/reference/functions/wp_authenticate/