If you confine use of the library to within the walls of your corporation, you do not have to distribute the source (even to your employees), because you are not redistributing (selling or giving away a software product that includes the library) outside of your organization.
The GPL allows you to freely use the code inside a corporation without restrictions, and that includes (by necessity) your ability to prevent your employees (as a matter of company policy) from distributing the source code outside the organization.
From the Gnu Licensing FAQ:
Is making and using multiple copies within one organization or
company “distribution”?
No, in that case the organization is
just making the copies for itself. As a consequence, a company or
other organization can develop a modified version and install that
version through its own facilities, without giving the staff
permission to release that modified version to outsiders.
However, when the organization transfers copies to other organizations
or individuals, that is distribution. In particular, providing copies
to contractors for use off-site is distribution.
Since you're making a derivative work of GPL-licensed software, your combined work would need to be licensed under the GPL as a whole. However, if you don't want to distribute your work, you are in no way compelled to shared your changes.
The GPL FAQ has this to say on keeping your modifications private:
Does the GPL require that source code of modified versions be posted to the public?
The GPL does not require you to release your modified version, or any part of it. You are free to make modifications and use them privately, without ever releasing them...
But if you release the modified version to the public in some way, the GPL requires you to make the modified source code available to the program's users, under the GPL.
Thus, the GPL gives permission to release the modified program in certain ways, and not in other ways; but the decision of whether to release it is up to you.
Thus, you are not required to make your modified program public, but you if you do, you must share it under the GPL.
Perhaps you're also worried that sharing your output calendars will compel you to share your modified code. The FAQ explains that output data produced by a GPL-licensed work is not covered by the GPL, unless the output actually contains GPL-licensed material (e.g., if part of the GPL-licensed program itself is included in the program's output):
Is there some way that I can GPL the output people get from use of my program? For example, if my program is used to develop hardware designs, can I require that these designs must be free?
In general this is legally impossible; copyright law does not give you any say in the use of the output people make from their data using your program. If the user uses your program to enter or convert his own data, the copyright on the output belongs to him, not you. More generally, when a program translates its input into some other form, the copyright status of the output inherits that of the input it was generated from.
So the only way you have a say in the use of the output is if substantial parts of the output are copied (more or less) from text in your program. For instance, part of the output of Bison (see above) would be covered by the GNU GPL, if we had not made an exception in this specific case.
In what cases is the output of a GPL program covered by the GPL too?
Only when the program copies part of itself into the output.
So, your output is probably not GPL-licensed. Note that even in some unusual case where the output is somehow GPL-licensed (like a Bison grammar), the output is a separate work from the program that created it. You'd need to obey GPL restrictions on the output, but sharing GPL-licensed output does not mean you must share changes on the GPL-licnesed program that created that output. The GPL on the program only comes into effect when the program itself is distributed, not when its output is distributed.
Best Answer
There are two cases in which it is possible to effectively change the copyright license on existing code.
The existing copyright license gives you the right to change the license or to sub-license the code. The GPL licenses do not fall in this category of licenses.
You are one of the copyright holders on the code and all copyright holders explicitly agree to the license change. If you ever accepted contributions from others without an explicit re-assignment of the copyright, then you the copyright on those contributions belongs to the person that wrote the contribution and you are bound by the license terms that were in effect when the contribution was made.
If you want to change the license, you must get permission from all contributers. If any one of them refuses permission, then you must either remove their contributions entirely or forget about the license change.
In both cases, the license change only applies to future versions.
If you don't fall in any of the two categories above, then you can not change the license.
There is one additional possibility to keep your code secret:
Copyright licenses govern what others can do with code you own. They are only relevant when you distribute the code (either in binary or source code form). If you don't distribute your own code, you don't need to bother to think about copyright licenses and if you use code owned by others but you don't redistribute it, then you only need to pay attention to the right to modify or make derived works. In all open-source licenses, any requirements on providing the source code is tied to distribution of your changes and don't apply if you don't distribute.
The key here thus is distribution of the program/library. Different licenses have different notions of what constitutes distribution, ranging from giving out copies to allowing remote access to the program.
But in all cases, if an organization makes tools for internal use, then allowing employees to use those tools is not considered distribution under copyright law. This means that you don't need to release the code for internal tools, even if those tools make use of GPL-licensed code.