If you are asking about the pronunciation of the “s” sound at the end six, dix, then yes, it is not pronounced in six janvier. For dix mars, I am not so sure. I could say it both ways, although without sounds better.

There are actually three possible situations, depending mainly on the word following six or dix:

1. If the following word starts with a consonant or an aspirated h, then the final consonant shouldn’t be pronounced. Examples : Le six mars, le dix mai, pronounced si and di.
2. If the following word starts with a vowel or a mute h, then the final consonant is pronounced as a (z). Example : Le six avril pronounced le sizavril.
3. In the other cases (mainly when six or dix are alone, or finishing a sentence, but also when they are used as a pronoun), the final consonant is pronounced (s). Example : Il m’en reste dix à faire (diss à faire). Le mot que tu cherches se trouve à la page six. (siss).
   Source : https://www.fpjq.org/pdfs/08-05_outils-nombres.pdf (along with my everyday experience of my native language).

To take in consideration: according to the region of France you live in, liaisons and pronunciation of a final letter are subject to change; in the south, people are more up to pronounce the final x of six & dix as an s, less in the northern half; there are even some regions (more in the north-eastern country) where the final t in vingt is being pronounced(but not the g), which always surprises me.