I'm not sure if this is specifically an Enterprise Guide question or not (I'm new to EG but am long-in-the-tooth on SAS).
Unless this is already solved, you may need to clarify it a little. For example you refer to two columns of dates but in your example I can only see one column.
Given a date eg 05May2006, what do you mean by "the rolling 12 months", do you mean 04May2007 or something more tricky, also, are you solely an EG point'n'click user or are you happy about writing a little datastep code?
This is pure Base SAS, not EG, so it will have to be pasted into a Code window (although it could also be constructed in a Query task), but I think it solves your problem:
informat payments best.
format date date9.;
input payments date;
create table rolling as
,sum(b.payments) as rolling_total
from example a
where b.date between a.date and mdy(month(a.date),day(a.date),year(a.date)-1)+1