The phrase "an unknown time" is correct and usable in written English.
It can be used when referring to a time that is not specified or is uncertain.
Example: "The event will take place at an unknown time, so please stay tuned for updates."
Alternatives: "an unspecified time" or "a vague time".
This week Greece and the eurozone entered an unknown time zone, of uncertainty and failure.
"So we are no longer dealing with an unknown time, date and place and that is very important".
A Kind Man is the story of a couple, Eve and Tommy, who live in an unnamed town, at an unknown time, not long ago.
Cornish, who also co-wrote last year's The Adventures of Tintin for Steven Spielberg, has been hired to take on a project described as a "high-octane adventure set in the prairie lands of an unknown time".
The basic technique, originated at an unknown time, was apparently practiced widely in Southeast Asia with local variations, as in Celebes Island, where the wax was applied with bamboo strips.
I need to tell the caterer our final numbers soon, and we have to finalize a seating chart for dinner, but how do I go about it with the possibility of having guests arrive at an unknown time?
A semilinear parabolic problem of second order with an unknown time-convolution kernel is considered.
A linear wave equation with an unknown time-convolution kernel is considered.
The MEMS gyroscope is considered with unknown parameters including mechanical structure imperfections and an unknown time-varying angular rate.
In the analyzed systems, the stock at a distribution center is used to fulfill an unknown, time-varying market demand.
In this paper, a semilinear parabolic problem with an unknown time-dependent source function p t) is studied.
