Hi @Glaga ,
Could you please share a bit about the data source used in your app? A SP List?
Do you store the validation of absence requests into your SP List?
Based on the issue that you mentioned, I think this issue is related to time zone. On your side, you should enable "Include time" option for date time type column in your SP List as below:

then when you generate a new app based on your SP List, the Date time column from your list would be decorated as a date time picker as below:

Set the Update property of the Date Time field data card to following:
DateValue1.SelectedDate + Time(Value(HourValue1.Selected.Value), Value(MinuteValue1.Selected.Value), 0)
Set the Items property of the HourDropdown box to following:
["00","01","02","03","04","05","06","07","08","09","10","11","12","13","14","15","16","17","18","19","20","21","22","23"]
Set the Items property of the HourDropdown box to following:
["00","01","02","03","04","05","06","07","08","09","10","11","12","13","14","15","16","17","18","19","20","21","22","23","24","25","26","27","28","29","30","31","32","33","34","35","36","37","38","39","40","41","42","43","44","45","46","47","48","49","50","51","52","53","54","55","56","57","58","59"]
In addition, you must make sure the Time zone setting and Locale setting of your SP Site have been set to same value as your local PC/machine:

Within your flow, when you want to create an event in requester's Outlook calendar, please configure your flow as below:

Within the "Create event (V4)" action, Start Time field set to following:
triggerBody()?['Created']
End Time field set to following:
triggerBody()?['DueDate']
Please consider take a try with above solution, check if the issue is solved.
Best regards,