Linker error with one particular function

[Gary Willoughby]

I have defined the following module:

/**
  * Module containing unit test helper functions for date time 
operations.
  */
module common.test.unit.datetime;

/**
  * Imports.
  */
import std.conv;
import std.datetime;

/**
  * Return a unix timestamp.
  *
  * Params:
  *     T = The return type of the unix timestamp.
  *     A = Variadic argument type for args parameter.
  *     args = Time arguments as specified by the DateTime struct.
  *
  * Reference:
  *     $(LINK http://dlang.org/phobos/std_datetime.html#.DateTime)
  */
T getUnixTime(T, A...)(A args)
{
	return to!T(SysTime(DateTime(args)).toUnixTime());
}

But when i import it and use the getUnixTime function i get the 
following linker error:

Undefined symbols for architecture x86_64:
   "_D6common4test4unit8datetime12__ModuleInfoZ", referenced from:
       _D5logic25generalstatisticprocessor12__ModuleInfoZ in main.o
       _D5logic14eventprocessor12__ModuleInfoZ in main.o
       _D5logic24devicestatisticprocessor12__ModuleInfoZ in main.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

Any idea why this is? If i comment out the getUnixTime function 
and put in other code it seems to import and work ok. Is it 
anything to do with the way i've written the function?