PERM - relieving letter start Date

[Raju1stnov]

I came to USA on 15-Jan-2015 on H1 visa to work for an employer X. That company did not pay me until 31-march-2015 for the pay period 01-Mar-2015 to 15-mar-2015. I had to leave the Employer X because of some reasons in Aug-2015.  Now my current employer Y is starting the GC process and for that I need to mention the start date with employer X.

Now for some reason the employer X is not ready to give me the relieving letter despite my repetitive requests. I have two questions.

1. Considering the above mentioned situation. What should be my start date with the employer X , is it 15-Jan-2015 which is the date I reached USA with employer X's petition or it should be 01-March-2015 The first date of the pay period.

2. What is the remedy with the relieving letter as I do not think they are going to provide me anyway.

I thank you very much for your help in advance.