What were the dates of the campaign that eventually led to the Treaty of Roskilde?
between 30 January and 8 February 1658
Title: Treaty of Roskilde
Passage: The Treaty of Roskilde was concluded on 26 February (OS) or 8 March 1658 (NS) during the Second Northern War between Frederick III of Denmark–Norway and Charles X Gustav of Sweden in the Danish city of Roskilde.  After a devastating defeat, Denmark-Norway was forced to give up a third of its territory to save the rest, the ceded lands comprising Blekinge, Bornholm, Bohuslän (Båhuslen), Scania (Skåne) and Trøndelag, as well as her claims to Halland.
Title: March Across the Belts
Passage: The March Across the Belts was a campaign between 30 January and 8 February 1658 during the Second Northern War where King Charles X Gustav of Sweden led the Swedish army from Jutland across the ice of the Little Belt to Funen and the Great Belt to reach Zealand.  The risky but vastly successful crossing was a crushing blow to Denmark, and led to the Treaty of Roskilde later that year, which handed Scania to Sweden.
Title: Joachim Gersdorff
Passage: Joachim Gersdorff (12 November 1611 - 19 April 1661) was a Danish politician, from 1650 to 1660 Steward of the Danish Realm.  It was Gersdorff who negotiated the Treaty of Roskilde on Denmark's part during the Second Northern War, a war he had himself been in favour of entering.  The treaty, which was concluded in Roskilde on 8 March 1658 (NS), ceded Scania, Halland, Blekinge and Bornholm to Sweden.
March Across the Belts Treaty of Roskilde