That's a superscript a, indicating that these are ordinal numbers rather than cardinal numbers.
Specifically, it's showing the ending of the word to disambiguate different ways of reading the number. 18 on its own would usually be read duodēvīgintī "eighteen", but with the -a ending marked, it has to mean duodēvīcēnsimā "eighteenth (day)".
In other words, as you correctly surmised, it's the same as writing 18th, to show that you mean "eighteenth" instead of "eighteen". The same convention is used in some modern Romance languages; in Spanish, for example, one could write día 18.ª for día decimoctava.
(For 1a the abbreviation is a bit less obvious, because ūnā "one (day)" would also end in -a. But the convention established from the other numbers, and from the context, is that the superscript ending means it's the ordinal prīmā "first (day)".)