Let \(\mathbb{I}\) denote the set of all compact intervals on the real line: \(\mathbb{I}=\{[a^-,a^+]\mid a^-\leq a^+\}\). For \(a,b\in\mathbb{R}\), we write \[ [[a,b]]= \begin{cases} [a,b]&\text{if \(a\leq b\)};\\ [b,a]&\text{if \(a>b\)}. \end{cases} \] The \textit{generalised Hukuhara difference} (\textit{gH-difference} for short) \(\ominus_{\text{g}}:\mathbb{I}\times\mathbb{I}\to\mathbb{I}\) is given by \[ [a^-,a^+]\ominus_{\text{g}}[b^-,b^+]=[[a^--b^-,a^+-b^+]]. \] We furnish \(\mathbb{I}\) with a complete metric \(D\) by setting \[ D([a^-,a^+],[b^-,b^+])=\max\{| a^--b^-|,| a^+-b^+|\}. \] The authors succeed in using these concepts to give a simpler definition of the differentiability of interval-valued functions. We say that \(f:(a,b)\to\mathbb{I}\) is \textit{generalised Hukuhara differentiable} (\textit{gH-differentiable} for short) at \(x_0\in(a,b)\) if \[ \lim_{h\to0}\frac{1}{h}\bigl(f(x_0+h)\ominus_{\text{g}}f(x_0)\bigr) \] exists. It turns out that the gH-differentiability is equivalent to the weakly generalised (Hukuhara) differentiability, which was previously defined in a rather complicated manner by looking at four cases separately. It can be shown that if we write \(f(x)=[f^-(x),f^+(x)]\), then \(f'(x)=[[(f^-)'(x),(f^+)'(x)]]\); therefore it is easy to compute \(f'\) for concrete functions \(f\). The paper then studies interval differential equations by using the gH-differentiation of interval-valued functions.