The Danish authorities are estimating that the risk of getting infections with the B1.351 mutation, also known as the South African mutation, from the border regions and into Denmark has increased.
As a result has the government decided to introduce two changes to the current rules of entry for people resident in the border areas when it comes to testing and self-isolation, entering into effect from tomorrow, the 17th of February 2021.
I will let the official description speak for itself as it's quite clear on its own:
(...) the Government has introduced stricter requirements for persons entering Denmark from the border regions, including Schleswig-Holstein, southern Sweden (the provinces of Blekinge and Scania) and western Sweden (the provinces of Halland and West Gothia). This entails that persons who enter Denmark with a worthy purpose must be able to present a negative COVID-19 test taken no more than 72 hours before entry. Under the rules applicable so far, the test must have been taken no more than seven days before entry.
The Government has also introduced stricter post-entry testing and self-isolation requirements for persons (also Danes) who arrive to work, provide services, visit a close relative or live in a border region. Under the rules applicable so far, it has been possible in some circumstances to derogate from the post-entry testing and self-isolation requirements if the relevant person presented a negative COVID-19 test taken no more than seven days before entry. This rule has been tightened to the effect that the test must have been taken no more than 72 hours before entry.
The changes will take effect on 17 February 2021. The rules applicable so far will remain in force until 17 February 2021.
The full statement can be found here: https://www.justitsministeriet.dk/pressemeddelelse/stricter-rules-on-entry-for-travellers-from-the-border-regions-as-of-17-february-2021/