If modified on same day, show only time

Give this a try:

if(get_the_modified_date('zY')==date('zY')) {  
the_modified_date('g:i a'); 
} else {
the_modified_date('F j, Y'); echo ' at '; the_modified_date('g:i a');
}

It checks that the modified date is the current day. If it is, it only displays the modified time. Otherwise it displays both the date and time.