(I should note that the "we" below is mostly the royal "we", expressed by the author, Jon Skeet. Hopefully most of the contributors to Noda Time agree with most of the below, but please don't assume that they all agree with everything.)
Noda Time has been designed with the following goals in mind:
We want to force you to think about decisions you really need to think about. In particular, what kind of data do you really have and really need? Is it local or global? Do you need the system time zone or some other time zone? If you're converting from a local time to a global one in a particular time zone, how do you want to handle ambiguities or gaps?
We want to solve the 99% case. Noda Time doesn't support leap seconds, relativity or various other subtleties around time lines. While we support some other calendars, we don't support you creating your own calendar - because you almost certainly don't need to. By focusing on the needs of the vast majority of users, we will make their lives easier... but this does mean that if you need something really specialized, Noda Time isn't likely to be right for you.
While we sympathize with the "make simple things easy, make complex things possible" motto, our experience is typically that in making complex things possible (with no indication that anyone actually wants to do those things), the simple things become that much harder.
We don't want to be your performance bottleneck. We regard Noda Time as a system-level library: we don't know exactly how you'll use it, or how performance-critical that use will be. We're willing to do more work (occasionally at the expense of internal complexity) to get out of your way, but we're not going to sacrifice public elegance for this.
We want your code to be robust in the face of new versions. Noda Time follows Semantic Versioning so you should be able to spot incompatible versions - but additionally, by limiting the amount of "hooks" we provide, we've reduced the opportunities for coupling between your code and ours.
We want your code using Noda Time to be testable. To some people that will conjure up images of interfaces and virtual methods everywhere - but that's not the case. Instead, we recommend that you inject appropriate dependencies (such as clocks) and we provide designed-for-testing implementations of types such as time zones.
We don't like defaults. Just about the only thing Noda Time will default for you is the use of the ISO calendar, as we believe that's what the majority of developers want. However, we do not default to using the system time zone, or using "now" as a default date/time value, or using the current thread's current format provider for parsing and formatting (except for the BCL-compatible method calls; see text handling for more information on this). We make it easy to do all of these things, but they're just not appropriate as implicit defaults. It's too easy to accidentally depend on the time zone your system happens to be running in (etc) without anything obvious in your code.
There are rather more types and concepts to learn about in
Noda Time than in .NET. One of the problems with .NET's date and
time API is that
DateTime doesn't have a single well-defined
There are more value types in Noda Time than in many other
libraries. We believe they're justified as value types (they
represent single values) but you need to be aware of the impact on
boxing and the like. In many cases the default value of the type (e.g.
new LocalDateTime() or
default(LocalDateTime)) is not a useful
value. This is unfortunate, but hard to avoid.
All the value types and almost all the reference types are immutable and thread-safe. We expect objects like calendars, time zones, and patterns for formatting and parsing text to be reused freely between many threads. Occasionally there's hidden mutability in terms of caches, but this should not affect you, the user. We make sure it all stays thread-safe for you.
Almost all types are sealed, and there actually aren't very many interfaces. Even the abstract classes often have internal abstract methods, so can't be derived from by your code. This follows Josh Bloch's approach of "design for inheritance or prohibit it" - inheritance takes a lot of effort to do robustly, and we don't want to break your code because you happen to rely on us calling a particular method at a particular time.
We're aware that this is one of the most contentious aspects of Noda Time's design - particularly as Joda Time is very extensible - but we believe that it's highly unlikely that you'll ever want to extend Noda Time anyway. We want to be an externally-simple library you can just rely on. If you have extra requirements, chances are someone else will want to do something similar - so pop along to the mailing list and we can collaborate on trying to solve your problem within Noda Time itself.