That sounds like a naive date->datetime conversion rather than explicit boundary exclusion. If you convert a date to a datetime, you get midnight at the start of day, which is fine if you're only comparing dates, but fails spectacularly in the way you describe for date ranges.

MSSQL not supporting a DATE data type has been the bane of my existence for a long time (of course, Postgresql has supported it since forever, but try telling your application vendor that). And since it took so long to include it, hardly any application uses it even today, because of inertia and compatibility with existing databases.