Question: Hell Creek Formation includes an isolated section that was designated a monument on which date?

Context paragraphs:
Hell Creek Formation: The Hell Creek Formation is an intensively-studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana.  The formation includes portions of Montana, North Dakota, South Dakota, and Wyoming.  In Montana, the Hell Creek Formation overlies the Fox Hills Formation.  "Pompey's Pillar" at the Pompeys Pillar National Monument is a small isolated section of the Hell Creek Formation.

Gilmoremys: Gilmoremys is an extinct genus of softshell turtle which lived during the late Cretaceous (Maastrichtian stage) of North Dakota, Montana and Wyoming, United States.  It is known from five skulls, a mandible and an incomplete postcranial skeleton.  The holotype USNM 6727 consists of a nearly complete carapace and an isolated hyoplastral fragment, was first assigned to the species "Aspideretes lancensis".  Many additional specimens were later discovered including cranial remains, and the material was assigned to its own genus, "Gilmoremys".  It was found from the Hell Creek Formation and from the Lance Formation (only the holotype).  It was first named by Walter G. Joyce and Tyler R. Lyson in 2011 and the type species is "Gilmoremys lancensis".  The generic name honors Dr. Charles W. Gilmore.

Protungulatum: Protungulatum ('first ungulate') is an extinct genus of mammal first found in the Bug Creek Anthills in northeastern Montana.  The Bug Creek Anthills were initially believed to be Late Cretaceous (latest Maastrichtian) because of the presence of the remains of non-avian dinosaurs and common Cretaceous mammals, but these were later shown to have been reworked from Late Cretaceous strata, and consequently the Bug Creek Anthills are currently believed to be Early Paleocene (Puercan) in age.  Remains from the Ravenscrag Formation of Saskatchewan, Canada have been assigned to "P. donnae".  These remains may also be Cretaceous in age, but the age of the Ravenscrag Formation is not entirely certain.  In 2011, remains of a new species of "Protungulatum", "P. coombsi", from the Hell Creek Formation, which is definitely Late Cretaceous (Maastrichtian) in age, proved that "Protungulatum" was present in both the Cretaceous and the Paleocene.  It was initially assigned to the order condylarthra, a group of archaic "ungulates", that is now known to be polyphyletic.  According to Archibald et al. (2011), "Protungulatum" is not even definitely a placental mammal.  Some studies have found it to be close to Cetartiodactyla, but the most recent analysis helds it to be a non-placental eutherian.

Pompeys Pillar National Monument: Pompeys Pillar National Monument is a rock formation located in south central Montana, United States.  Designated a National Monument on January 17, 2001, and managed by the U.S. Bureau of Land Management, it consists of only 51 acre , making it one of the smallest National Monuments in the U.S.  It was previously designated a National Historic Landmark on July 25, 1965.  The new Pompeys Pillar Interpretive Center opened in 2006.  Exhibits in the 5,700-square foot center relate the journey of Captain William Clark and his detachment, including Sacagawea and her son Pomp, down the Yellowstone River Valley in 1806.

Vladimír Socha: Mgr.  Vladimír Socha (born January 1, 1982 in Hradec Králové) is a Czech writer, publisher, public lecturer and science promoter from the city of Hradec Králové (north-eastern Czech Republic).  His main interest lies in dinosaur paleontology and the history of science in general.  So far, he published eight books about dinosaurs and primeval life in Czech language, four smaller brochures (booklets) and many magazine articles about nature, history and prehistory.  In 2009 he volunteered in the summer paleontological field dig in the Hell Creek formation in eastern Montana (for the "Museum of the Rockies" in Bozeman).  A year later, he published a book about the experience called "Dinosauři od Pekelného potoka" (Dinosaurs from the Hell Creek).  He also provides interviews for the Czech Radio and sometimes organises paleoart exhibitions.

Odaxosaurus: Odaxosaurus is an extinct genus of anguid lizards that existed in western North America from the Late Cretaceous to the Paleocene.  Fossils of the type species "Odaxosaurus piger" and the species "O. priscus" are widespread throughout Late Cretaceous formations in the western United States and Canada.  First described in 1928 from the Lance Formation in Wyoming, "O. piger" has since been found in the Hell Creek Formation in Wyoming and Montana, the Frenchman and Scollard formations in Alberta, and the Aguja Formation in Texas.  It was one of the few species of lizards to survive the Cretaceous–Paleogene extinction event, which is estimated to have killed off 83% of all lizard species.  The second species, "O. priscus", was named in 1996 from the Dinosaur Park Formation in Alberta and has since been found in the Kaiparowits Formation in southern Utah.  Remains of an anguid from the Kirtland Formation in New Mexico may also belong to "Odaxosaurus".

Lance Formation: The Lance (Creek) Formation is a division of Late Cretaceous (dating to about 69 - 66 Ma) rocks in the western United States.  Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the latest Mesozoic vertebrate faunas.  The Lance Formation is Late Maastrichtian in age (Lancian land mammal age), and shares much fauna with the Hell Creek Formation of Montana and North Dakota, the Frenchman Formation of southwest Saskatchewan, and the lower part of the Scollard Formation of Alberta.

Cimolopterygidae: Cimolopterygidae is a group of extinct birds known from the late Cretaceous Period.  Remains attributed to cimolopterygids have been found in the Frenchman Formation of Saskatchewan, the Lance Formation of Wyoming, the Fox Hills Formation of Colorado, the Hell Creek Formation of Montana, and the Allen Formation of Rio Negro, Argentina.  Most date to the end of the Maastrichtian age, about round 5 million years ago, though a much earlier species has also been identified from the Campanian-aged Dinosaur Park Formation of Alberta, about 75 million years ago.

Coriops: Coriops is a genus of prehistoric bony fish.  Its fossils are found in Campanian (Mesa Verde Formation), Maastrichtian (Hell Creek Formation), and possibly Paleocene (also Hell Creek) age deposits.  This chronological distribution means that the genus may have survived the Cretaceous–Paleogene extinction event that killed the dinosaurs.

Acheroraptor: Acheroraptor is an extinct genus of dromaeosaurid theropod dinosaur known from the latest Maastrichtian Hell Creek Formation of Montana, United States.  It contains a single species, Acheroraptor temertyorum.  "A. temertyorum" is one of the two geologically youngest known species of dromaeosaurids, the other being "Dakotaraptor", which is also known from Hell Creek.

Supporting paragraphs: Hell Creek Formation Pompeys Pillar National Monument
Answer: January 17, 2001