From GridLAB-D Wiki
Integers requiring 64 bits are stored in an int64 built-in data type. An int64 value can represent values between −9,223,372,036,854,775,808 to 9,223,372,036,854,775,807.
An attempt to store a value outside that range is unpredictable.
GLM
To declare an int64 value in a class use the syntax
class my_class { int64 my_string; }
To define an integer value in an object use the syntax
object my_class { my_integer 123; }
where "value" is a integer within the range of a 64 bit integer.