@Override
    public List<TimeZone> parseValueCollection(String value) throws GuacamoleException {
        
        if (value == null)
            return null;
        
        // Split string into a list of individual values
        List<String> stringValues = Arrays.asList(DELIMITER_PATTERN.split(value));
        if (stringValues.isEmpty())
            return null;
        
        // Translate values to Integers, validating along the way.
        List<TimeZone> tzValues = new ArrayList<>();
        for (String stringTz : stringValues) {
            tzValues.add(parseValue(stringTz));
        }

        return tzValues;
        
    }
A regex that matches valid variants of GMT timezones.
guacamole-ext/src/main/java/org/apache/guacamole/properties/TimeZoneGuacamoleProperty.java