I have a betterment account. It allows you to have a personalized automatic target date fund. Basically you set a date (only goes in yearly time frames for now), and the fund will automatically adjust from stocks to bonds as you close in on your liquidation date. FWIW, on the backend it also optimizes for taxes, with your entire portfolio . This is , I think, the best way of saving for short term goals.
If you don't want a betterment account to do this, you could also do it manually. But I also think it is better, behaviourally, to have it automated.