Thanks for your replay mr.Danilo Bambi ,
I will tell you what i want exactly in Excel.
I have an one custom form.

As shown in the picture above, there is Worker no : 000001 against Valid from : 2/1/2006.
The standard HcmWorker form.
Worker no : 000001 against in the Employment history valid from 2/1/2006.
Note : in my custom form i did it manual entry.
The same process while import in excel and 'valid from' and 'valid to' null in excel column based on a worker against in the employment valid from date insert in my custom form valid from.
Now, i will go to import worker no :000011 in Employment history valid from (8/5/2009 12:00:00 AM)
The same valid from while import if valid from null same date i want.
My excel column:

Valid from , valid to is empty once imported 000011 record.
Then i will go to check my custom form. there is worker against in the emmployment history same to date insert or not.
So that how to write in x++ give me example for that.
Thanks for your attention.