The EO is confusing to me what does it mean that "cannot make punishable an act prior to Jan 1st 2019" Full EO

https://www.whitehouse.gov/presidential-actions/2018-amendments-manual-courts-martial-united-states/

Sec. 2.  The amendments in Annex 1 shall take effect on the date of this order, subject to the following:

(a)  Nothing in Annex 1 shall be construed to make punishable any act done or omitted prior to the date of this order that was not punishable when done or omitted.

(b)  Nothing in Annex 1 shall be construed to invalidate the prosecution of any offense committed before the date of this order.  The maximum punishment for an offense committed before the date of this order shall not exceed the maximum punishment in effect at the time of the commission of such offense.

(c)  Nothing in Annex 1 shall be construed to invalidate any nonjudicial punishment proceeding, restraint, investigation, referral of charges, trial in which arraignment occurred, or other action begun prior to the date of this order, and any such nonjudicial punishment proceeding, restraint, investigation, referral of charges, trial in which arraignment occurred, or other action shall proceed in the same manner and with the same effect as if the amendments in Annex 1 had not been prescribed.