Date column + Time Column

The entry in your first row makes no sense. If your time component represents Seconds, the time component of the first row is over ten days, yet your expected data shows the result to be on the same day.

If you really want to ignore multiple days in the End column, then you can use in your custom column formula:

=DateTime.From(Number.From([Date]) + Number.Mod([End],86400)/86400)

If you really want to keep the dates where End implies more than one day, then remove the Number.Mod function:

=DateTime.From(Number.From([Date]) +[End]/86400)

Also, note that, unlike Excel, you cannot specify a particular format for a Date column in Power Query (You can in a Power BI visual). In Power Query, the appearance will be that specified in your computer Windows Regional Settings.

If you really require the format to be as you show in your Excel expected output example, you may need to convert it to a text string. Of course, if you do that, date-related math operations will be much more difficult.

Here's an example:

=DateTime.ToText(DateTime.From(Number.From([Date]) +[End]/86400), "dd/MM/yyyy hh:mm tt")