escaping should be added to list of hardest things in computer science

typescript

[dongcarl@dai tar-testing]$ REFERENCE_DATE=2019-01-01
[dongcarl@dai tar-testing]$ REFERENCE_TIME=00:00:00
[dongcarl@dai tar-testing]$ export TAR_OPTIONS="--mtime="$REFERENCE_DATE\\\ $REFERENCE_TIME""
[dongcarl@dai tar-testing]$ echo $TAR_OPTIONS
--mtime=2019-01-01\ 00:00:00
[dongcarl@dai tar-testing]$ tar -cf in.tar in/
[dongcarl@dai tar-testing]$ tar tvf in.tar
drwxr-xr-x dongcarl/dongcarl 0 2019-01-01 00:00 in/
-rw-r--r-- dongcarl/dongcarl 0 2019-01-01 00:00 in/hello
[dongcarl@dai tar-testing]$ unset TAR_OPTIONS
[dongcarl@dai tar-testing]$ rm in.tar
[dongcarl@dai tar-testing]$ tar --mtime="$REFERENCE_DATE\\\ $REFERENCE_TIME" -cf in.tar in/
tar: Substituting -9223372036854775807 for unknown date format ‘2019-01-01\\\\ 00:00:00’
[dongcarl@dai tar-testing]$ echo "$REFERENCE_DATE\\\ $REFERENCE_TIME"
2019-01-01\\ 00:00:00
[dongcarl@dai tar-testing]$ tar tvf in.tar
drwxr-xr-x dongcarl/dongcarl 0 -9223372036854775808 in/
-rw-r--r-- dongcarl/dongcarl 0 -9223372036854775808 in/hello
[dongcarl@dai tar-testing]$ exit