You can achieve this by setting the attribute for your input minuteValuesas follows:
<ion-datetime displayFormat="HH:mm" [(ngModel)]="myDate" minuteValues="0,30">
minuteValues can be an array of numbers or a string of comma-separated numbers.
EDIT:
. ion-datetime Ionic 1.x, Ionic 2.x. Ionic 1.x , ion-datetime-picker