Display Date of Joining on PA20/PA30 Screen
SPRO >> Personnel Management >> Personnel Administration >> Customizing User Interfaces >> Change Screen Header
--> Header Modifier
Header IDs are assigned a header modifier, from which the structure of the respective headers is defined, as well as the HR country modifier and transaction class in table T588I Assignment of Header ID/ Country Grouping to Header modifier.
00 - Initial screen of transactions PA20, PA30, PA51 and PA61, PB20, PB30, etc.
10 - Infotype 0000 – Actions, Infotype 4000 - Applicant Actions
03 - Infotype 0001 - Organizational Assignment
01 - Infotype 0002 - Personal Data
04 - All infotypes of Time Management
02 - All other infotypes
As my scenario is to display field on PA 20, PA30 Screen so I have chosen ‘00’ Screen header, and gave unique header modifier for reference to next step
--> Field names different to DDIC entry
One of the following field labels "short keyword", "middle keyword", "long keyword", or "different keyword from DDIC" can be assigned to a screen field (in the header)
This field ‘name’ is displayed on the fronted screen i.e. PA20, PA30
--> Infotype header definition
Select the header modifier which we have selected on the previous step eg – ‘09’
Check the available space to accommodate the required field by performing a simple test 
so that we can get grid view on the screen header, eg as below, By this single test we can get the view of screen on which line and column space is available
As per below image available space is in Line 2, column 45 onwards.
Infotype contains a code that represents a specific infotype. Infotypes allow you to describe object attributes
Field name refers to the attributes which needs to be displayed.
Once the record is created, then test it through 
Adjustment of column can be done by shifting the column number if required
through this action we can check for any error in configuration of screen headers
Once the output is fine then 
After generating, check the PA20/PA30 screen, changes would be available now.
Regards,
Dhiren