Want to enter 12 March 2021 i enter it press enter reverts to 11/03/2021
What is an “English date” anyway, different English speaking countries write dates in different forms.
However, 12 March 2021 and your 11/03/2021look like en-GB.
Obvioiusly you entered the “English date” - and Calc understood. This seems to indicate that you are using an English (UK) locale or one insofar equivalent.
If yo want to see something precisely as you typed it despite the fact that it is recognised as numeric data (what dates are in spreadsheets), you need to either
-1- mark it explicitly as text by prefixing a single apostrophe
or (better)
-2- set the ‘Numbers’ format of the cell you want to enter it to ‘text’(code @) in advance.
Otherwise a recognized date is always displayed in the (bad) default format of the locale which you cannot change or in the ‘Numbers’ format for dates you defined for the cell - best by assigning a respective cell style. The format code for what you want should be DD MMMM YYYY.
But: You might also consider to use the one and only globally well understood date format YYYY-MM-DD (ISO 8601) instead of sticking to any local format. There are many advantages.
I doubt that the input of 12 March 2021 yields a date 11/03/21, but more likely it yields 12/03/2021. So to display the date in the long form you prefer, assign the cell the number format DD MMMM YYYY under Format → Cells… (Ctrl+1) → Numbers.
I’m using English(UK) for UI and locale. It handles the input as the questioner stated. (V 7.1.1.1)
(All the locales I checked for years now, have default date formats with 2-digit-year and all kinds of additional nonsense.)
Update:
There was something mixed up (by me?)
The actual format shown after having entered 12 March 2012 is 12/03/21 for me. This is not exactly how the questioner reported.