The freshness seal that we mention in the datasheet means that the battery is left unconnected until the first time Vcc is applied to the device. The 10-year lifetime of the battery will not start until Vcc is first applied and the battery is enabled. This means that if you ordered the DS1225 two years ago, but just supplied power to the device this year, the battery should last for another ten years rather than eight years.
The battery will have a small self-discharge rate. The battery inside the DS1225, the BR1225 does not list the self-discharge rate in the datasheet, but it is usually around 0.3% per year. This means that each year, the battery will lose 0.3% of its capacity even if power is not applied. This is very small compared to the overall capacity of the battery and should not have a big impact on the lifetime of the battery if it is only left sitting for a few years.