The answer is that it depends. Often a furlough is to an unsecured local treatment facility, and if that is the case you don not have a right to credit. A judge may still grant you credit, but you are not entitled to it, and such a grant is uncommon.
On the other hand, if your were at a secured facility, such as AMRTC, it will count as in custody time and youw ill get credit. On a 21 month sentence you would ordinarily serve 14 months in custody, and 7 months on supervised release. So you are close to have the sentence satisfied.
I assume that the furlough happened before you were sentenced. A 21 month sentence if executed, would committ you to the commissioner of corrections, at which point the district court would loose jurisdiction. On the other hand, a 21 month stayed sentence, with local jail time, and a treatment furlough is a common outcome.
I suggest that you talk through the details with a criminal defense attorney, because there are a few technical issues that need to be clarified.
The answer provided is not intended as legal advice. Any question within a case involves numerous facts and circumstances that cannot be fully accounted for in this forum. You are strongly encouraged to contact an attorney to discuss your issue fully. John C. Conard can be reached for a free consultation at 651-264-3050.
Generally the answer is no. You only get custody credit if you are in custody. Treatment is not in custody. In some rare cases, a treatment facility can be considered a custodial situation because you are not free to leave and it is a locked facility, but as I said this is rare.
This answer is not legal advice and does not create an attorney/client relationship. You need to speak to a lawyer to discuss your specific legal situation and to receive legal advice.