What you need to know
A timestamp represents a moment in time, while a birthday or appointment date may be a calendar value in a particular timezone. Converting them with the wrong assumptions can shift a day for users.
How to apply it
Store instants in a consistent timezone such as UTC, preserve timezone context when it matters, and format dates for the user's locale at display time. Test around midnight and daylight-saving transitions.
Common mistake to avoid
Do not parse arbitrary date strings and assume every browser treats them identically. Use a documented format and validate user-entered dates.