A query for 12 month rolling values

[Maxim de Winter]

Hello,

could you please help me with setting up a query to get 12 month rolling values? I found some threads here reagrding this but the solutions proposed never worked for me.

I have 3 columns:

1)
Mth_Year (I have it as text, having used the TEXT function in Excel)
01/11
02/11
...
08/12

2) Country
Poland
Hungary
Czechia...

3)
Number or orders


As of 01/12 I would like to have another column showing me the 12mth rolling sum of the number of orders for each country. Is there is way to do this without an SQL code?
I have tried DSum function combining it with Dateadd function but either gave back a syntax error or the summed figures were wrong (too high).

Could the date format be possibly a problem? Should I use something as a substitude instead?

Thanks a lot in advance for your help!

Max