Better to use an explicit conversion of string-date to actual date, than hope that local settings will accidentally give you the result you want with an implicit conversion.
CONVERT(DATE,po_date,101)
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Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden