We could use some more information...
"I have a timesheet set up with a filtered portal. The user selects the "Month/Yr" from the global field object on a form layout,"
The user selects the value from a drop down menu? What exact value do they select "4/2009" or such?
Is the global field a date field or a text field?
then proceeds to the portal on the same form to record time worked each day.
Does the user press a key (tab/return/enter) or use the mouse to click into the portal? What happens if you click a blank part of the layout after selecting a field in the global value?
When the drop-down calendar is selected, the current month pops up rather than the month selected in the global field. What was visible in the portal and what value was present in the global field before the user selected a month/year?
How do the records that you expect to appear in the portal get there in the first place?
Please describe your tables, their relationships, the key field's data types and whether there are any calculations involved.
How do I get the proper month to appear when the calendar drop-down is selected?
It sounds like what you've done should work. Obviously it doesn't so we have to take a careful look at the picky details that control how a relationship and the portal function.