Great question, this one looks really interesting from an expiration and enforcement perspective.
Usually, calculating an expiration date for a patent is straightforward. You can use the Patent Term Calculator Excel sheet from the USPTO, as described here. This spreadsheet works in this case, but there are some interesting things about this one that are worth noting.
The issue date of this patent was April 22, 1997. From the specification:
This application is a continuation-in-part of application Ser. No. 08/231,035 filed Apr. 22, 1994, now abandoned.
The above indicates a claim of domestic benefit (in this case, a continuation-in-part) from an earlier application, which sets the priority date of this patent to April 22, 1994. This, in itself, isn't unusual, and it effectively limits the term of the patent by establishing an earlier priority date while potentially limiting the earliest date the claims are enforceable (unknown without opening the provisional).
The filing date of the application (Appl. No. 08/537,541) for the patent grant (October 2, 1995) is also important, because it was shortly after June 7, 1995, when a revised set of rules went into effect, potentially providing a second type of Patent Term Extension (PTE) under 154 (in addition to PTE 156), while the provisional was filed prior to that date. I'm going to have to look in to this part a little more in order to provide a correct interpretation of the actual effect of this on the patent term. I believe this patent did not have a PTE, but I haven't verified that yet.
This document is actually an Ex-Parte Re-Examination Certificate (US 5,623,660 C1), issued October 4, 2011 based on Re-Examination Request No. 90/010,636, filed December 16, 2009.
AS A RESULT OF REEXAMINATION, IT HAS BEEN DETERMINED THAT:
Claims 1, 2, 8, 18, 22-27 and 42 are cancelled.
New claims 49-94 are added and determined to be patentable.
Claims 3-7, 9-17, 19-21, 28-41 and 43-48 were not reexamined.
The above re-examination effectively altered the enforceability of the original claims, but added 45 new claims (I assume that these are based on the 11 cancelled claims in the original grant, but narrowed based on the re-examination). I'm tempted to work through these, but it probably won't add anything to the conversation; however, (pure speculation) this re-examination and the subsequent claims allowed may have occurred based on information added during the continuation-in-part vs. what was available in the original provisional application.
If my understanding of the priority rules for continuation-in-part are correct, then at the date of the issue of the C1 document, the revised claims are in effect and enforceable from that date until the end of the original patent term, which would be April 22, 2014. However, the original claims are in effect from (depending on the content of the provisional specification vs. the continuation-in-part specification) either April 22, 1994 or October 2, 1995 until October 5, 2011.
That is the best I can do from my recollection of the rules, and I'm not that confident of that last paragraph. If nobody else provides a more complete answer in a day or two, I will look into it a bit more. The parts I need to research are in italics.
I'm certain there is a wealth of interesting reading material in the Public Pair Image File Wrapper for this patent. I'll revise this answer when I have more time to read through it.
Note: The reason I'm so interested in when the specific claims were enforceable is that if a past infringement were identified within the term of the claims, it could still result in damages being awarded. More specifically, the amended claims awarded from the re-examination resulted in an amendment of at least one independent claim.