Bit time only calculates the ejection of one bit, it looks at how this bit transmits out of a network interface card at a determined speed i.e.
bit time = $1 / Bandwidth$
Let Bandwidth be 10Mbps, then bit time = $1 / (10 * 10^{6})$$1 / (10 * 10^{6}) \rightarrow 10^{-7} seconds$