Article: Kathmandu is home to a number of museums and art galleries, including the National Museum of Nepal and the Natural History Museum of Nepal. Nepal's art and architecture is an amalgamation of two ancient religions, Hinduism and Buddhhism. These are amply reflected in the many temples, shrines, stupas, monasteries, and palaces in the seven well-defined Monument Zones of the Kathmandu valley are part of a UNESCO World Heritage Site. This amalgamation is also reflected in the planning and exhibitions in museums and art galleries throughout Kathmandu and its sister cities of Patan and Bhaktapur. The museums display unique artifacts and paintings from the 5th century CE to the present day, including archeological exportation.

Now answer this question: From what time period are the oldest artifacts in Kathmandu museums?
5th century