If you look at the raw date values, you should see they look like those shown in the screenshot. The expression column is: [Date,0]. These are just floating point numbers. MC does date conversion upon display, so your dates will be expressed in the locale's format, and locale should not affect the internal value of the date.
Possibly when you tried to assign the date field
"=[Date]+300" or "=[Date]-300"
you modified the type from a floating point to a string.