This is my solution in Odoo 18.0. I think it same on 17.0

/** @odoo-module **/

import { DateTimeField, dateRangeField } from "@web/views/fields/datetime/datetime_field";

import { patch } from "@web/core/utils/patch";

import { onMounted, useRef } from "@odoo/owl";

const { DateTime } = luxon;

patch(DateTimeField.prototype, {

setup() {

super.setup();

this.startDateRef = useRef("start-date");

onMounted(() => {

this.startDateRef.el.addEventListener("focus", this.onFocus.bind(this));

});

},

/**

* @param {PointerEvent} ev

*/

onFocus ({ target }) {

if(this.props.defaultStartDateField === "today" && !this.state.value[0]) {

this.state.value = [

DateTime.now(),

false

];

}

},

});

DateTimeField.props = {

...DateTimeField.props,

defaultStartDateField: { type: String, optional: true },

};

const super_extractProps = dateRangeField.extractProps;

dateRangeField.extractProps = ({ attrs, options }, dynamicInfo) => ({

...super_extractProps({ attrs, options }, dynamicInfo),

defaultStartDateField: options.default_start_date_field,

});

In the views, you can add it to options
<field name="date_start" position="attributes">

<attribute name="options">{'end_date_field': 'date_end', 'useCurrent': 'day'}</attribute>

</field>