nope, you are not using the right classname,
in your example you must use something like:
<ion-popover class="popoverDatetime" trigger="open-date-input" show-backdrop="false" [dismissOnSelect]="true" side="top" alignment="start" size="cover" >
<ng-template>
<ion-datetime #popoverDatetime presentation="date" [max]="maxDate"
(ionChange)="targetDate = popoverDatetime.value;">
</ion-datetime>
</ng-template>
</ion-popover>