Move the latest or older File from one directory to another Directory

I Need help for one requirement,

I want to move the latest/Older file in the folder to another file. File have the datetimestamp in postfix.

Example:

Source Directory : \a
destination Directory : \a\b
File1 : xy_MMDDYYYYHHMM.txt (xy_032120101456.txt)
File2: xy_MMDDYYYYHHMM.txt (xy_032120101856.txt)

i want to move latest/older file which is File2 need to destination folder. need to achive this in shell script.

Any idea? Please. Thanks in advance.

If they are always named "xy_" something, then

cp `ls -ut xy_* | head -1` b/

works. Or if it's always 2 character underscore and date, then

cp `ls -ut ??_* | head -1` b/