%% Compile and read me!
\documentclass[a4paper,12pt]{article}

\usepackage{fmtcount,etoolbox}\def\x{\the\numexpr}\def\n[#1]{\numberstringnum{#1}}\def\h[#1]{\ifnumequal{#1}{0}{twelve night}{\ifnumequal{#1}{12}{twelve noon}{\ifnumless{#1}{13}{\n[#1] o'clock AM}{\n[\x#1-12\relax] o'clock PM}}}}\def\m[#1]{\ifnumequal{#1}{15}{quarter}{\ifnumequal{#1}{30}{half}{\n[#1] minutes}}}\newcounter{c}\def\f[#1]#2{\ifnumequal{#2}{0}{\h[#1]}{\ifnumless{#2}{31}{\m[#2] past \h[#1]}{\setcounter{c}{\x60-#2\relax}\m[\thec] to \h[\x#1+1\relax]}}}

\begin{document}

\noindent
\textbf{Test cases:} \\ \\
00:00 = \f[00]{00} \\
12:00 = \f[12]{00} \\
06:00 = \f[6]{00} \\
18:00 = \f[18]{00} \\
06:15 = \f[6]{15} \\
18:45 = \f[18]{45} \\
11:30 = \f[11]{30} \\
13:22 = \f[13]{22} \\
17:43 = \f[17]{43} \\

\noindent
\textbf{More test cases:} \\ \\
00:13 = \f[00]{13} \\
12:12 = \f[12]{12} \\
12:15 = \f[12]{15} \\
11:45 = \f[11]{45} \\
11:41 = \f[11]{41} \\

\end{document}