function toMillis(timestamp, picture) {
        // undefined inputs always return undefined
        if(typeof timestamp === 'undefined') {
            return undefined;
        }

        if(typeof picture === 'undefined') {
            if (!iso8601regex.test(timestamp)) {
                throw {
                    stack: (new Error()).stack,
                    code: "D3110",
                    value: timestamp
                };
            }

            return Date.parse(timestamp);
        } else {
            return parseDateTime.call(this, timestamp, picture);
        }
    }
Converts an ISO 8601 timestamp to milliseconds since the epoch

@param {string} timestamp - the timestamp to be converted
@param {string} [picture] - the picture string defining the format of the timestamp (defaults to ISO 8601)
@returns {Number} - milliseconds since the epoch