Bug: DateTimeUtils.UTCToString returns local time despite the "UTC" name (also: deprecated substr)

## Summary `DateTimeUtils.UTCToString` is named and documented as returning a UTC time string, but it returns the local-timezone string via `Date.prototype.toTimeString()`. Anyone consuming this output has to know to ignore the function name, and tests/CI on machines in different timezones can produce different results. ## Affected code `lib/DateTimeUtils.ts:3-7` ```ts public static UTCToString(UTCString: string) { let utcDate = new Date(); utcDate.setUTCHours(+UTCString.substr(0, 2), +UTCString.substr(2, 2), 0); return utcDate.toTimeString(); // <-- returns LOCAL time string } ``` `toTimeString()` produces e.g. `"08:30:00 GMT-0700 (Pacific Daylight Time)"` for an input that was conceptually 15:30 UTC. ## Suggested fix Either return a UTC representation: ```ts return utcDate.toUTCString(); // or, just the time portion: return utcDate.toISOString().slice(11, 19); ``` …or rename the method if the local-time behavior is intentional. ## Related smaller cleanup The whole class uses `String.prototype.substr()`, which is [a legacy/deprecated API](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr). It still works but should be replaced with `substring()` or `slice()` for consistency with the rest of the codebase (`convertHHMMSSToTod`, `convertDayTimeToTod`, etc., already use `substring`). Affected: `UTCToString`, `UTCDateTimeToString`.