How to Sum up 2 fields of integer, and present as "Time period"?
One way to do this is the Computed Field module, which will allow you to use PHP to perform calculations.

To use this, you could add a field, Total time, of type Computed Field.  Then you could write some PHP code to calculate the value for Total time based on the other two fields.  This way, every time the node is updated, the correct calculation value will be saved in the Total Time field.

If you need to customize the display output, the Computed Field module allows you to optionally add PHP code to output the field for display with custom formatting as well.

But

Please note the warning on the module's page:


  This module is intended to be used by PHP developers to insert &amp;
  execute PHP code on a Drupal website. It does not provide a macro
  interface, a code builder or anything other than insert your working
  code here into this text area. You need to have a good working
  knowledge of PHP and the Drupal API to use this module. To get the
  best results with this module, pre-test your PHP code in the Body
  field of a Page in Drupal.