zeitgeber とは、ドイツ語で英語でいうとtime giverだそうです。time giver timeというとますますわからないですね。zeitgeberのzeitはtimeというよりも時間周期を与えるキュー（刺激）みたいに捉えたほうがしっくりきます。
“Zeitgeber” in German is “Time giver” in English and refers to the “agent of entrainment”. https://ruo.mbl.co.jp/bio/e/product/circadian/exuse/per1-2.html
下の説明が非常にわかりやすく、徹底的です。
A zeitgeber is any environmental cue that can be used by an organism to align its endogenous rhythm with the external day-night cycle. So far we have focused mainly on light as it is the most common zeitgeber, but there are other zeitgebers as well.
the experimental manipulations that chronobiologists impose lead to two new ways of talking about time. When an organism is free-running, we speak of circadian time (CT). When an organism is entrained to an artificially-imposed, environmental daily cycle, we speak of zeitgeber time (ZT).
the peak of mRNA abundance (phase) Peak times (phase) and amplitudes are summarized in circle plots. the peak times (also referred to as phases) of pre-mRNA accumulation for cycling genes https://www.pnas.org/doi/10.1073/pnas.1715225115 この論文では位相（遺伝子発現のピーク）をZT(zeitgeber time）で表示しています。
phase (the maximum peak of expression of a certain gene) https://stackoverflow.com/questions/30463959/circadian-phase-plot-in-r
