+ * Chooses a prefix so that the value in front of the decimal point is between 1 and 999.
+ */
+SIPrefix determine_value_prefix(double v);
+
+/**
+ * Formats a given timestamp with the specified SI prefix.
+ *
+ * If 'prefix' is left 'unspecified', the function chooses a prefix so that
+ * the value in front of the decimal point is between 1 and 999.
+ *
+ * The default value "s" for the unit argument makes the most sense when
+ * formatting time values, but a different value can be given if the function
+ * is reused to format a value of another quantity.
+ *
+ * @param t The value to format.
+ * @param prefix The SI prefix to use.
+ * @param precision The number of digits after the decimal separator.