If no time is specified, it defaults to 00:00:00
touch -d "2010-05-31" a.txt
touch -d "2010-05-31 08:10:30" a.txt
Only modify modification time
touch -m -d "2010-05-31" a.txt
Only modify access time
touch -a -d "2010-05-31" a.txt
The time format can be written without double quotation marks. Personally, it is better to write it.
touch --help
Usage: touch [options]...files...
change the access time and modification time of each file to the current time.
Files that do not exist will be created as empty files unless the -c or -h option is used.
Special treatment if the filename is "-", changing the access time of the file relative to standard output.
Parameters that are required for long options are also required for short options.
-a change access time only
-c, --no-create do not create any files
-d, --date=string use specified string for time instead of current time
-f (ignore)
-h, --no-dereference Affects the symlink itself, not the destination indicated by the
symlink (this option is only useful when the system supports changing the owner of a symlink)
-m only change the modification time
-r, --reference=file uses the specified file's Time attribute instead of the current time
-t STAMP Use the time in [[CC]YY]MMDDhhmm[.ss] format instead of the current time
--time=WORD Use the time specified by WORD: access, atime, use are all equal to
the -a option effect, and modify, mtime is equal to the effect of the -m option
--help display this help information and exit
--version display version information and exit
Note that the -d and -t options accept different time/date formats.