T = 12.4 h, x(t) = .250 d (displacement)
The total amplitude is half of the distance d from the highest to lowest level.
Using the equation x(t) = x(m) cos(wt + φ), we get:
.250d = .5d cos(wt + φ)
To find w, use the equation w = 2π / T
.250d = .5d cos((2π / 12.4)t + φ)
.250d = .5d cos((2π / 12.4)t + 0)
Solve the equation for t.
.250d / .5d = cos ((2π / 12.4) t)
arccos(.5) = 2π / 12.4 * t
arccos(.5) * 12.4 / 2 / π = t
t = 2.066666 hours