Tracking a Date & Time Field's History

I found the following article to be incredibly helpful for creating a field that tracks the Name History in Sugar. http://support.sugarcrm.com/Knowledge_Base/Studio_and_Module_Builder/Sugar_Logic/Sugar_Logic_Walkthrough_Tracking_a_Fiel… 

I want to create a similar field that tracks any change made to a date/time field called "Last Phone Call Date"

The original formula for tracking changes made to a name looks like this: 

ifElse(contains($name_history_c,concat($first_name," ",$last_name)),  $name_history_c,concat($name_history_c," ",$first_name," ",$last_name))

I would think the following changes would work to track changes made to "Last Phone Call Date" but I get this error message when I click save: concat: All parameters must be of type 'string'

Did I simply mess up the formula OR can a date/time field not be copied and tracked using a text area field?

ifElse(contains($history_of_calls_logged_c,concat($last_phonecall_c)),$name_history_c,concat($history_of_calls_logged_c," ",$last_phonecall_c))