Magento 2 - How to add the DateTime UI Component

To add the dateTime picker you should use the next directive inside the xml file:

<field name="start_date">
    <argument name="data" xsi:type="array">
        <item name="config" xsi:type="array">
            <item name="dataType" xsi:type="string">string</item>
            <item name="label" xsi:type="string" translate="true">Go Live Start Date</item>
            <item name="formElement" xsi:type="string">date</item>
            <item name="source" xsi:type="string">page</item>
            <item name="sortOrder" xsi:type="number">21</item>
            <item name="dataScope" xsi:type="string">start_date</item>
            <item name="validation" xsi:type="array">
                <item name="required-entry" xsi:type="boolean">true</item>
            </item>
            <item name="options" xsi:type="array">
                <item name="dateFormat" xsi:type="string">yyyy-MM-dd</item>
                <item name="timeFormat" xsi:type="string">HH:mm:ss</item>
                <item name="showsTime" xsi:type="boolean">true</item>
            </item>
            <item name="storeTimeZone" xsi:type="string">string</item>
        </item>
    </argument>
</field>

Important thing is the showsTime option.

Result should look like this:

If you want to debug the UI element - use this command inside the browser console (on your page):

require('uiRegistry').get('index = start_date')

It returns your date element with all initial options and all possible functions:

You can use them when you define the element (inside xml).

As additional info:

The date (also dateTime) element could be found here:

app/code/Magento/Ui/view/base/web/js/form/element/date.js

in the static files:

pub/static/adminhtml/Magento/backend/en_US/Magento_Ui/js/form/element/date.js

The date-element class (object) has method prepareDateTimeFormats, where the important option showsTime is checked:

/**
 * Prepares and converts all date/time formats to be compatible
 * with moment.js library.
 */
prepareDateTimeFormats: function () {
    this.pickerDateTimeFormat = this.options.dateFormat;

    if (this.options.showsTime) {
        this.pickerDateTimeFormat += ' ' + this.options.timeFormat;
    }

    this.pickerDateTimeFormat = utils.normalizeDate(this.pickerDateTimeFormat);

    if (this.dateFormat) {
        this.inputDateFormat = this.dateFormat;
    }

    this.inputDateFormat = utils.normalizeDate(this.inputDateFormat);
    this.outputDateFormat = utils.normalizeDate(this.outputDateFormat);

    this.validationParams.dateFormat = this.outputDateFormat;
}

I hope this answer gives some idea about what you miss

I added the UI component of date time field through php (it works fine)

$fieldset->addField(
        'event_date',
        'date',
        [
            'name' => 'event_date',
            'label' => __('Date'),
            'title' => __('Date'),
            'required' => true,
            'date_format' => 'yyyy-MM-dd',
            'time_format' => 'hh:mm:ss'
        ]
);

compare to your xml file all values are present except date_format and time_format so you can try to set the both value in your xml file

for more details please refer this full source code