I assume that you have an inject node connecting to the function node and that inject node is sending a timestamp as msg.payload. If you change the inject node to send a string, you will see that spring replacing the time stamps.
When you have an issue it is wise to provide a simple flow showing the issue.
In order to make code readable and usable it is necessary to surround your code with three backticks (also known as a left quote or backquote ```)
```
code goes here
```
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See this post for more details - How to share code or flow json
