<?php
// How many times per day, get from DB
$userInput = 17;
$SecondsPerDay = 60 * 60 * 24;
$Frequency = (int) ($SecondsPerDay / $userInput) -1;
$midnight = strtotime('today midnight');
// Get an array of times to execute
$executeTimes = array();
for($time=$midnight; $time<=($midnight+$SecondsPerDay); $time+=$Frequency)
$executeTimes[] = date("g:i a", $time);
print_r($executeTimes);