DateTime control doesn't correctly evaluate date value that do not exactly match date-format of site locale
DateTime Calculated value control evaluates to current date. If it happens that a date value is entered into DateTime control that doesn't exactly match date-format pattern of the locale of the site, then the referenced calculated value control's evaluates to incorrect date. The input might be in a format that can be be recognized as a date, so the control accepts (and validates) the input, however, the value evaluates to current date.
Update to the latest version of the product. This has been fixed on May - 2018.
If there is no time to update, then the workaround is to ensure that there is a space between, days, months and years when entering a date value Nintex form in a site with Slovak locale. Alternatively pick a date from the date control.