In Syr: the abs. is yōm/yūm and the most common pl. is the emphatic form yawmāṯā, while the absolute and construct have the masculine plural ending as does the emphatic following numbers; OA ymt, BA יֹומָתָא ; elsewhere ya/ōm, yaw/ōmīn; there appears to be a lexical distinction between the two plurals in the older dialects, with yawmīn referring to literal "days," while yawmāṯā refers rather to "time, times," but they can be used interchangeably even in the same text: see Hugoye 14(2011):55.

The Mandaic expression "riding on the day" undoubtedly reflects the common Akkadian expression RKB ūmē said of the gods "riding on a storm wind" (see Müller-Kessler, "Aramäische Beschwörungen und astronomische Omina ...", pp. 433-35), but the word iuma was surely always simply understood as "day" within Mandaic.
As for the expression using the repeated plural in the KTMW inscription, we believe that the commonly agreed upon interpretation of similar expressions in the MT as "yearly" is unwarranted and should be rendered "from time to time" and would so suggest is the best understanding of the KTMW passage as well.