2018.2 changes the default date format in account preferences to

M/D/YYYY

. Since (in my experience) SuiteScript 2.0 requires you to set date fields using a Date object created from a string in the account preferred date format, this could be a breaking change for scripts that set date values. Because of the above, I was using a function to detect the user account preference and format the date accordingly

var fmt = runtime.getCurrentUser().getPreference({ name: 'DATEFORMAT' });

but this returned

fmMM/DDfm/YYYY

instead

M/D/YYYY

, which broke my script anyway.