This cannot ever work. Give it up. Only the ISO yyyy-mm-dd format is unambiguous.
If a user sees some random date, such as 3/4/xxxx, they have absolutely no way to distinguish if this is in their format or not.
If you're in the US, you might assume that this is in your backwards M/D/Y format, meaning it's April 3rd, and you'd be right much of the time.
However, if you're outside the US, then there are two indistinguishable possibilities:
It's a US server ignoring your time format, so this date represents April 3rd.
OR
It's a server correctly showing you a date in your preferred format, so it's actually the 4th of March.
At a glance, these aren't distinguishable. I easily click through 200-500 pages a day! I'm not going to sit down and try to work out which of those hundreds of pages a day, and thousands of pages a week are designed for humans, or just Americans.
If a user sees some random date, such as 3/4/xxxx, they have absolutely no way to distinguish if this is in their format or not.
If you're in the US, you might assume that this is in your backwards M/D/Y format, meaning it's April 3rd, and you'd be right much of the time.
However, if you're outside the US, then there are two indistinguishable possibilities:
It's a US server ignoring your time format, so this date represents April 3rd.
OR
It's a server correctly showing you a date in your preferred format, so it's actually the 4th of March.
At a glance, these aren't distinguishable. I easily click through 200-500 pages a day! I'm not going to sit down and try to work out which of those hundreds of pages a day, and thousands of pages a week are designed for humans, or just Americans.