empty – How to display – unknown values ​​(date)

My product is a software / application of condominium management.

My application allows condominium management staff to enter and post a period of time during which a unit will be empty (the resident of the unit goes on vacation, uses it as a residence secondary school, etc.). For example, staff may enter a "vacancy". "From March 1, 2019 to March 15, 2019. However, a survey of users indicates that condominium staff will not always know what is the return date of a given resident. direction may notice that a resident is not or has not been there for a few days and may want to enter the unit in a "vacant" area, but does not know when the resident will return, so our application allows this user to enter a start date and will not ask him to enter the "end date." However, I do not know how to best display an "end date n / a" in the different areas of our interface user where the system would indicate the end date.For example, see here: https://cldup.com/J_tzuLYg-n.png

I do not necessarily want to post something like "n / a" or "unknown", that is to say:

  • March 1, 2019 – n / a
  • March 1, 2019 – date unknown

I would like to post something more human and friendly.

Thank you very much and sorry for the long question.