below is the output is ls -l

-rw-r--r-- 1 tonyt staff 3212 Apr 17 1999 file1
-rw-r--r-- 1 tonyt staff 4541 Mar 3 21:08 file2

why the date format is not the same?

Since file1 was last modified in a year different than the current one, it will show you the year, instead of the time.

Then how can I change the mtime of a file? which command should I use?

Use the "touch" command.

You can use it to set the file timestamp to a time that you specify.

See touch(1) man pages for more information.

I've found another file:
-rwxr-xr-x 1 root wheel 3824 Apr 27 2001 env
So the year is current year. Why it show up the year instead of date?thx

On HP-UX, end of calendar year is not the cutoff. Datestamps older than approx 180 days are displayed with the year instead of the time. To demo this:

touch -t200105201200 myfile1
touch -t200105251200 myfile2
touch -t200105301200 myfile3
touch -t200106051200 myfile4

ls -l myfile*

Cute! It works. Thanks for your help.