Currently PowerApps doesn't have a time picker (or a date and time picker) - feel free to vote for some of the current feature requests in the PowerApps Ideas board for that to increase the priority for the team.
For now, you can use a pair of dropdown controls (Hour and Minute) to let the users specify the time. They will have their Items property defined as such:
DropdownHour.Items: ["00","01","02",...,"22","23"]
DropdownMinute.Items: ["00","01","02",...,"58","59"]
If you don't want that granularity for minutes, you can also have only certain values, such as ["00", "05", "10", ..., "50", "55"]. If you want to create a Time value from those controls, you can use the expression
Time(
Value(DropdownHour.Selected.Value),
Value(DropdownMinute.Selected.Value),
0)
Or if you have a date picker and you want to create a date/time value (which you then can use to compare against date/time columns):
DatePickerDate1.SelectedDate +
Time(
Value(DropdownHour.Selected.Value),
Value(DropdownMinute.Selected.Value),
0)