I’m using date format: 1999-12-31. What do I type in to get: 2017-03-13?
First: your question title (using / as date parts separator) differs from body of question (referring to - as separator).
Second: if you use the date format mentioned in title, then you will get the wanted (in title) result automatically.
The title should be a summary of the question, and then provide more details in the question body. See guidelines for asking.
It does not say where you are from in the profile settings, but for my locale (en-us), it works like this.
Select cell A1 and press CTRL+1 to show these format settings.
Now enter 3/13/17 to produce this result.
What needs to be entered varies according to the Date acceptance patterns specified at Tools → Options → Language Settings → Languages.
Direct input of an ISO date like 2017-03-13 is accepted in every locale, independent of current date acceptance patterns, and automatically applies the ISO date format. It is not necessary to pre-format the cell with the ISO format.