The schedule parameter is very flexible, but must take one of the following forms to be interpreted correctly:

An ISO-8601 formatted date or date-time string (e.g. YYYY-MM-DD or YYYY-MM-DD HH:MM:SS in 24-hour UTC time)
An expression of the form "in 5 minutes" where intervals supported are minutes, hours, days and months.
A date expression like "5 days from now" or "tomorrow"
A crontab-compatible specification like 0 * * * * (every hour at 0 on the hour) or 0 0 * * * (every day at midnight). This also includes directives like @hourly or @monthly.
A repeating expression like "every 3 hours" or "every day at 1am"

In all cases times are UTC.