Is it allowed for しち reading of 7 to be used in compound numbers?


I thought that [七]{しち} (7) and [九]{く} (9) readings could not be used in
constructing compound numbers, like 71 or 19, and unlike their alternative
readings, [七]{なな} (7) and [九]{きゅう} (9), can only be used for numbers 7 and 9.
But now I noticed that both readings of 7, [七]{しち} and [七]{なな}, can describe
17th and 27th day of month. What's more, only [九]{く} (9), not [九]{きゅう}, is
used to describe 19th day of month. What am I getting wrong?




You're not getting anything wrong. Cardinal numbers are read one way, and the
days of the month are read another way. Just try to memorize the "irregular"
readings for the days of the month, as well as the ones for the months of the
year, minutes, hours, and counters in general. I find repeating them aloud
helps you get them down faster.

