helpers: Update datetime function

Remove the process.env.TZ check - we explicitly set this
Add a comment about setting this value

src/helpers/datetime-helper.js

@@ -1,10 +1,9 @@
 const { DateTime } = require('luxon');
 
 function getDateTime(ianaTimezone, dateString, timeString) {
+  // Node pulls the timezone from the system on initialisation using the TZ environment variable.
+  // We can change process.env.TZ to UTC. This will update the current Node process.
   process.env.TZ = 'UTC';
-  if (process.env.TZ !== 'UTC') {
-    throw new Error(`Schedule generation logic relies on 'TZ' env var being set to 'UTC'. It is currently: ${process.env.TZ}`);
-  }
   if (typeof dateString !== 'undefined' && typeof timeString !== 'undefined') {
     return DateTime.fromISO(`${dateString}T${timeString}`, { zone: ianaTimezone }).toJSDate();
   }