In Mahadevan's sign number 179, the symbol for a period of time is surrounded by four points of darkness, one for each cardinal direction. Each point of darkness signifies that something is false. In other words, for an unspecified period of time, something is not possible. It translates as 'this time cannot'.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-314 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 78: Collections in India: Suomalainen Tiedeakatemia.
Comment:
There is only one extant example of this sign in the Indus inscriptions.
Image Credit:
Indus Script Sign Number 179: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.