Basically, it will select all rows where the begin-date is before or on the 31st of January 2017, and the end date is on that date or after. If you are doing standard SQL, that is (in SAS, that happens if you write Pass-through SQL calls, or if the where clause is passed directly to an external DBMS). But, if you are doing SAS SQL, it can also be the opposite, if the begin date is larger than the end date. Including if the end date is missing, and the begin date is after the date in question. SAS has a problem with missing values in where clauses, and with the between condition.
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