From Xojo Documentation

Property (As Integer )
aXojo.Core.Timer.Tolerance = newIntegerValue
IntegerValue = aXojo.Core.Timer.Tolerance

Supported for all project types and targets.

A hint to the system as to how precise (in milliseconds) you need the timer to be, although you may get less tolerance that this depending on what the operating system supports. A value of 0 (the default) means to use the standard for the platform. A value of 100 asks for 100 millisecond tolerance.