When working with dates in Day.js, you can create date objects from strings in several formats. The most reliably supported format is ISO 8601, which follows the pattern "YYYY-MM-DDTHH:mm:ssZ". Day.js will correctly interpret ISO 8601 strings without any additional configuration.

Day.js also allows you to parse dates from custom string formats, such as "DD/MM/YYYY HH:mm", by providing the format string as a second argument. To use custom formats, you must include the customParseFormat plugin. This approach gives you flexibility to handle dates in various forms, but you must ensure the format string matches the input.

Parsing locale-specific or natural language date strings, like "December 31, 2023", is possible by specifying the format explicitly. However, Day.js does not automatically detect locale-specific formats; you must provide the exact format pattern.

If you use an unsupported or ambiguous format without specifying the pattern, Day.js may not parse the date correctly. Always prefer ISO 8601 for maximum compatibility, and use custom parsing only when necessary and with the correct plugin enabled.