Are you sure you don't want a report for this? A subform connotes data enrtry. If you want to display a certain period of time, you would do that in a report.
In any event, base the subform on a query, and put the following criteria in the period field:
BETWEEN [Forms]![frmYourMainFormName].[FromPeriod] AND [Forms]![frmYourMainFormName].[ToPeriod]
When the subform opens, it should contain only those values in the period.
Be sure to set the Parent Child link on the subform to the Account No.
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