def convert_to_internal(self, agenda):                                          
    x = []                                                                      
    t = OrderedDict(sorted(agenda['timconf'].items(),                           
                           key=TimeSerializer.__day_comparator))    
    for day, items in t.items():    

First of all, from what I can see, you're only using t once: in the loop.

Instead, I'd suggest writing

    t = sorted(agenda['timconf'].items(), key=TimeSerializer.__day_comparator)
    for day, items in t:

That gets rid of the OrderedDict.

Let's look at the following piece of code:

            for attr in ['begin', 'end']:                                       
                if hasattr('item', attr):                                       
                    item[attr] = item[attr] + day_num*24*60*60   

What does this do? Of course! It shifts an item by a given number of days.

But it also does some funny things. It checks if item has an attribute (.attribute), but then modifies the value at a key ([key]). So item is some weird attribute dictionary-ish thing. Let's use all that knowledge together.

def shift_time_item(time_item, num_days):
    delta = num_days * 24 * 60 * 60
    try:
        time_item.begin += delta
    except AttributeError:
        pass

    try:
        time_item.end += delta
    except AttributeError:
        pass

Now, you might think it is not DRY, because I repeat a similar try/except, but it is. And it does not really matter, because the business logic is there only once: I specify how to move an item just once.

        day_num = TimeSerializer.DAYS_INVERSE[day]                              
        for item in sorted(items, key=lambda k: k['begin']):
            shift_time_item(item, num_days)                                   
            x.append(TimeItem(**item))

Now, I'm left wondering what to do about the x. Why not

items = sorted(items, key=lambda k: k.begin)
for item in items:
    shift_time_item(item, num_days)

And then use items instead of x.

    p = ';'.join(map(str, items))                                                   
    return "-t {} -n {} -p '{}'".format(agenda['timeZone'], agenda['locale'], p)