def aggregate_conversations(li, delta):
    """Aggregates the conversation count.
    
    Keyword arguments:
        li --- The list of tuples containing tweet creation date
            and number of user mentions.
        delta --- The timedelta object by which the conversations
            should be aggregated.

    """

    count = []
    date = []

    index = -1

    for (d, c) in sorted(li):
        if index == -1:
            index += 1
            count.append(c)
            date.append(d)
        else:
            if d - date[index] <= delta:
                count[index] += c
            else:
                date.append(d)
                count.append(c)
                index += 1
    return (count, date)