addHours() can be used to add one hour to a Date Time field using Trigger in Salesforce.
Sample Trigger:
trigger AccountTrigger on Account( before insert ) {
for ( Account objAcc : trigger.new ) {
if ( objAcc.Start_Date_Time__c != null ) {
objAcc.End_Date_Time__c = objAcc.Start_Date_Time__c.addHours( 1 );
}
}
}
Output: