Overstaying a country or regional visa or visa equivalent can result in denied entry as well as revocation of future travel to the area.
Voyage Immigration tracks the number of days you spend in countries and regions ensuring that you don't overstay your visa. The app monitors the number of days and lets you know how much longer you can stay in the current country or region.
For example the ESTA for the USA allows you to stay in the USA for up to 90 continuous days. Should you overstay this period you might be removed from the country, but you will probably also lose the right to use the ESTA system going forward.
The visa free and regular business travel visa for the EU Schengen area allows you to stay within the Schengen countries for up to 90 days in any 180 day period.
EU citizens have the right to travel and work within any EU country, without restrictions. The free movement of people is a key pillar of the EU. With Brexit occurring in March 2019 it is essential for EU and UK residents to manage the number of days they spend in the other's area. British citizens will only be able to spend 90 days in the EU in any 180 day period, and EU citizens will have a similar restriction within the UK. Business traveller and leisure travellers who spend a significant time on the other area will need to be much more careful about their travel.