Enabling history timestamps makes old commands show .bash_history modified timestamp

[james harvey]

When I turn on timestamps for history, .bash_history records without a
timestamp (and possibly in-memory records without a timestamp - not
sure, haven't tested) inherit the modified timestamp of .bash_history
next time it is read.

This obviously causes old commands to have a much newer date.

I look at HISTTIMEFORMAT as display formatting only, since it writes
to .bash_history with since-epoch time.

My fix would be to log since-epoch timestamps for each command in
.bash_history, and if someone has default HISTTIMEFORMAT, just not to
show the timestamps.

I'd also implement a backup fix, because you can't retroactively get
timestamps for previous commands.  And, failing my original fix, this
one would make the situation better.  I'd have .bash_history commands
without a timestamp report as timestamp 0 - 1/1/1970 - rather than the
.bash_history modified date, to at least be clear something's off,
rather than stealthly report a wrong timestamp.

---Full history customizations---
==========
shopt -s histappend
export HISTSIZE=-1
export HISTFILESIZE=-1
export HISTTIMEFORMAT="%Y-%m-%d %H:%M:%S "
export HISTCONTROL=ignorespace
==========