Editing a date+time field converts date to text

Hello,

I have formatted a (Excel 2010 spreadsheet) column as
"ddd\ dd-mmm-yy\ \ hh-mm\ \h\r\s"
which shows the existing data as "Sun 04-Oct-15 17-30 hrs" format.

When I try to edit the date or the time, the cell content shows as "04 10 2015 05:30:00 PM". So far so good.

Now when I try to change the date or time (say, change it to 5-Oct), the data in the cell seems to convert to text !!!

Whatever I try, this does not convert to date format except if i delete the time portion (retaining the date portion). I have never encountered this earlier; have been updating this file since the year 2010.

Can someone help me please?