Oh, that is strange. I have not added any custom css, it's just a generated page. Could you perhaps share your project? Maybe I can find what's different.
Sure:
$('#AvailabilitySlot_TimeStart').timepicker({ minuteStep: 15, defaultTime: '9:00 AM', showSeconds: false, showMeridian: false, snapToStep: true });
and HTML:
<div class="form-group m-form__group col-3">
<label for="AvailabilitySlot_TimeStart">@L("TimeHour")</label>
<input class="form-control" placeholder="@L("TimeStart")" id="AvailabilitySlot_TimeStart" type="text" name="timeStart" readonly />
</div>