Merging Partition states moving the data into an exiting partition or to new file group (Forward only means to next file group or Partition).
The merged partition resides in the filegroup that originally did not hold boundary_value. boundary_value is a constant expression that can reference variables (including user-defined type variables) or functions (including user-defined functions). It cannot reference a Transact-SQL expression. boundary_value must either match or be implicitly convertible to the data type of its corresponding partitioning column, and cannot be truncated during implicit conversion in a way that the size and scale of the value does not match that of its corresponding input_parameter_type. (BOL)
Alter partition function easydb_partition_yearly_function()
merge range(‘2008-01-31 23:59:59.997’)