How to say your leaving and don't know when you'll be back?

I am joining the military and need to tell the people closest to me but I am scared to tell them. My father is needing surgery and would have it when I'm at basic and then I don't know how to tell my boyfriend, in the 5 years we've been together we have only been apart for about a week or so. I know this is what I'm suppose to do is serve in a bigger capacity than I am now.
How to say your leaving and don't know when you'll be back?
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