my discourse site is showing invalid date
This is likely a system administrator issue rather than a Discourse one. Is the date on the server correct? Somewhere like stack exchange is probably a better place to get help.
My system time is working fine
Then you’ll need to say more about where you are seeing the time and in what sense it is invalid.
actully my pc discourse is working fine but the mobile discourse is broken maybe it is not loading css… and its also showing Invalid Date in it.
How was this instance installed? Where is it installed?
Can you share the URL?