Reference: al-Muqarrab li-Ahkaam al-Janaa.iz – Question 143, Page 122
Question: During the period of mourning, is it permissible to wear a watch for time keeping and not for beautification?
Response: Yes, that is permissible for her, because the action follows the intention. However, it is preferable for her to refrain from doing so as it resembles jewellery.
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu `alayhi wa sallam) and his family and his companions.