Date table is throwing negative numbers in PowerPivot

Hi experts,

Hope I'm the right place. I have really looked in many dozens of webpages, forums and manuals but I cannot find anything like this.

Basically, I have 4 tables connected by Power Query in a schema that works perfectly. Data types and relationships are ok. Date columns have been set to my locale (es-CO) mostly, with the format date/time (d/m/yyyy hh:mm:ss) except for birthdays.

I created then in Power Pivot a "Date Table" but even before connecting it to the data model, I realised about two funny things:

• The function for the day number of the week, weekday() is throwing #ERROR: "An argument of the function has the wrong data type or the result is too large or to small."
• The calendar built starts in 1/1/1899 and 2077. 

Because of this, I tried then with these DAX formula:
=DATE(year('Calendar'[Date]);month('Calendar'[Date]);day('Calendar'[Date]))
and dates were converted to "1/1/3799 12:00:00 a.m." 

and 

=int('Calendar'[Date]) and they start with negative numbers (-363) 

What's going on?! I'm completely lost.

Thanks, Gerónimo.