All I can say is that she *should* just get retro to date of filing (so make sure you put that aside)... but judges sometimes do whatever the hell they want. In my state, CS is until the child is 18 or finishes high school, but MUST stop at age 19 even if they are still in school. (This is to prevent a BM from holding a child back a year or two to get an additional year's support.) My oldest SD didn't finish HS until she was 19. On her 19th birthday, DH (with a damn lawyer) went to get CS reduced. The judge decided it would be lowered on the date of her graduation from HS. Which is fucking illegal, but he did it anyway. Since it was "only" about 4 months worth of support, it would have cost more to fight it, so we didn't. Guess my state is pretty "pro-BM" too!

Here in Florida, my coworker got zinged for retro from date of promotion not date that she filed. But this is Florida

I believe she can only get from the date of filing.

From NYS Domestic Relations Law 240:

(j) The order shall be effective as of the date of the application therefor, and any retroactive amount of child support due shall be support arrears/past due support and shall, except as provided for herein, be paid in one lump sum or periodic sums, as the court shall direct, taking into account any amount of temporary support which has been paid. In addition, such retroactive child support shall be enforceable in any manner provided by law including, but not limited to, an execution for support enforcement pursuant to subdivision (b) of section fifty-two hundred forty-one of the civil practice law and rules. When a child receiving support is a public assistance recipient, or the order of support is being enforced or is to be enforced pursuant to section one hundred eleven-g of the social services law, the court shall establish the amount of retroactive child support and notify the parties that such amount shall be enforced by the support collection unit pursuant to an execution for support enforcement as provided for in subdivision (b) of section fifty-two hundred forty-one of the civil practice law and rules, or in such periodic payments as would have been authorized had such an execution been issued. In such case, the courts shall not direct the schedule of repayment of retroactive support. Where such direction is for child support and paternity has been established by a voluntary acknowledgement of paternity as defined in section forty-one hundred thirty-five-b of the public health law, the court shall inquire of the parties whether the acknowledgement has been duly filed, and unless satisfied that it has been so filed shall require the clerk of the court to file such acknowledgement with the appropriate registrar within five business days. Such direction may be made in the final judgment in such action or proceeding, or by one or more orders from time to time before or subsequent to final judgment, or by both such order or orders and the final judgment. Such direction may be made notwithstanding that the court for any reason whatsoever, other than lack of jurisdiction, refuses to grant the relief requested in the action or proceeding. Any order or judgment made as in this section provided may combine in one lump sum any amount payable to the custodial parent under this section with any amount payable to such parent under section two hundred thirty-six of this article. Upon the application of either parent, or of any other person or party having the care, custody and control of such child pursuant to such judgment or order, after such notice to the other party, parties or persons having such care, custody and control and given in such manner as the court shall direct, the court may annul or modify any such direction, whether made by order or final judgment, or in case no such direction shall have been made in the final judgment may, with respect to any judgment of annulment or declaring the nullity of a void marriage rendered on or after September first, nineteen hundred forty, or any judgment of separation or divorce whenever rendered, amend the judgment by inserting such direction. Subject to the provisions of section two hundred forty-four of this article, no such modification or annulment shall reduce or annul arrears accrued prior to the making of such application unless the defaulting party shows good cause for failure to make application for relief from the judgment or order directing such payment prior to the accrual of such arrears. Such modification may increase such child support nunc pro tunc as of the date of application based on newly discovered evidence.

If I am not mistaken, it goes back to the date she filed. Nothing before that.

What kills me is if DH loses his job and then gets another one at a lower salary (let's say from $100k to.$80k). CS will not be reduced in Massachusetts because he is CAPABLE of making $100k.

I believe that CS is important and that the laws are there for a reason but Massachusetts is particularly tough on fathers when it comes to CS.