modelform

class TimeLogForm(forms.ModelForm):
    class Meta:
        exclude = ['begin_sec']
        model = TimeLog

    def __init__(self, *args, **kw):
        super(TimeLogForm).__init__(self, *args, **kw)

    def save(self, *args, **kw):
        instance = super(TimeLogForm, self).save(commit=False)
        last_entry = TimeLog.objects.all().order_by('-id')[0] #filter by id here
        if last_entry:
            instance.begin_sec = last_entry.begin_sec
        else:
            # this is the very first record do something
            pass
        instance.save()
        return instance