As you have the association to the day object that contains the start and end times you can compare on those.
So this will be something like:
$Programitem/MyFirstModule.Programitem_Day/MyFirstModule.Day/endtime >= $Programitem/endtime and $Programitem/MyFirstModule.Programitem_Day/MyFirstModule.Day/starttime >= $Programitem/starttime
As for the validation feedback this is a specific action that allows you to set a validation message under on of the fields on a page and the message is defined by you. So you could also choose to use 2 decisions and check the starttime in one and the endtime in the second decision and make sure to set the validation messages under the appropriate field when the validation fails.