ExpressionEngine's Localize class currently lacks a method to modify a timestamp while keeping the member's localization settings in-tact. Adding this method around line 90 to to the file /system/expressionengine/libraries/Localize.php fixes that issue. Usage: $tomorrow = ee()->localize->modify_timestamp('+1 day');

ee_modify_timestamp.php

        /**
         * Given a modification like +1 day or -2 weeks and a Unix timestamp, 
	 * returns the modified timestamp in the specified timezone or member's
	 * current timezone.
	 *
	 * @param	string	Modificiation like +1 day
	 * @param	int		Unix timestamp
	 * @param	bool	Return date localized or not
	 * @return	string	Formatted date
	 */
	public function modify_timestamp($modify, $timestamp = NULL, $localize = TRUE)
	{
		if ( ! ($dt = $this->_datetime($timestamp, $localize)))
		{
			return FALSE;
		}
		
		return $dt->modify($modify)->format('U');
	}