if 0 {[Richard Suchenwirth] 2006-02-12 - One of [emacs]'s many features
is that when you have, in the first five (or so) lines of a file, the
string
 Time-stamp: "xxx"
the "xxx" part will be replaced by a time-stamp, every time you save the
file. Of course one could also tell from [file mtime], but if you have
e.g. undated printouts, such time-stamps can help tell which is the
newer. As Sunday is fun project time, I tried to emulate that feature
"in 10 LOC or less". I needed 10, but here it is, usable for a [text] widget:}
 proc timestamp w {
    set token {Time-stamp: "}
    set pos [$w search $token 1.0 5.end]
    puts pos:$pos
    if {$pos eq ""} return
    set from [$w index $pos+[string length $token]c]
    set to [$w search \" $from 5.end]
    puts from:$from,to:$to
    if {$to eq ""} return
    $w delete $from $to
    $w insert $from [clock format [clock sec] -format %Y-%m-%d,%H:%M:%S]
 }
#-- For testing, hit <F1> (in practice one would call ''timestamp'' before saving)
 pack [text .t]
 .t insert end "# Demo\n# Time-stamp: \"whatever\"\n# more text..."
 bind .t <F1> {timestamp %W}

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[Arts and crafts of Tcl-Tk programming]