Function coerceSince

coerceSince(value: string): string
Coerces a date string to the start of day (midnight) and returns ISO string with timezone.

Takes a date string (typically YYYY-MM-DD or ISO format) and normalizes it to the beginning of that day (00:00:00.000). Useful for ensuring date range queries start at the very beginning of the specified date.
Parameters
- value: string
  The date string to coerce (ISO format or YYYY-MM-DD)
Returns string
ISO string representing midnight on the specified date
Throws
If the date string cannot be parsed as a valid ISO date
Remarks
This function is used in the CLI to normalize the --since parameter, ensuring that calculations start at the exact beginning of the day in the schedule's timezone.

behaviour
- Parses input as ISO string
- Sets time to 00:00:00.000 (start of day)
- Returns full ISO format with timezone
- Throws error if parsing fails (fail-fast approach)
Example
```
coerceSince('2024-08-01');
// Returns: '2024-08-01T00:00:00.000+01:00' (or relevant timezone)

coerceSince('2024-08-01T14:30:00Z');
// Returns: '2024-08-01T00:00:00.000Z'

coerceSince('invalid-date');
// Throws: Error: Invalid date format: invalid-date. Expected ISO format (YYYY-MM-DD or YYYY-MM-DDTHH:mm:ssZ).
```
- Defined in DateUtilities.ts:110