This follows the same law as Shabbos in which the law is that one may lengthen the current status of the timer but not shorten it. However according to those Poskim<a href="//51076249-AFA5-4A3E-BB8A-23F8D605F103#_ftn1" name="_ftnref1">[1]</a> who permit Gerama on Yom Tov, seemingly they would allow even shortening the current activity due to Gram Havara.
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<a href="//51076249-AFA5-4A3E-BB8A-23F8D605F103#_ftnref1" name="_ftn1">[1]</a> Achiezer 3:60; Klei Chemda Vaykhel; Chasam Sofer Even Haezer 1:20; Chelkas Yaakov 1:50; Yabia Omer 3:17; SSH”K 13:27

Shulchan Aruch HaRav | May one adjust a Shabbos clock on Yom Tov?

This follows the same law as Shabbos in which the law is that one may lengthen the current status of the timer but not shorten it. However according to those Poskim[1] who permit Gerama on Yom Tov, seemingly they would allow even shortening the current activity due to Gram Havara.