The Microsoft .NET environment has a rich class library that you can access when developing custom indicators and strategies. There is a plethora of information available online and in print that details class libraries in great depth. Below are quick links to the Microsoft Developers Network for some of the basic classes whose functionality you may harness when developing in NinjaScript.
Complete list of classes in the Microsoft .NET environment.
Provides constants and static methods for trigonometric, logarithmic, and other common mathematical functions.
Full list of member of the System.Math class.
Represents an instant in time, typically expressed as a data and time of day.
Full list of members of the Sytem.DateTime structure.
Represents text; that is, a series of unicode characters.
Full list of members of the System.String class.