It looks like $birthyear is undefined. Try modifying birthdays.tcl as follows:
<br>
<pre>
if { ![empty_string_p $birthyear] } { # <-- change != to use empty_string_p function
if { $before_or_after_today == "0" } {
set age_on_next_birthday [expr $this_year - $birthyear]
append html " (turns $age_on_next_birthday today!)"
} elseif { $before_or_after_today == "-1" } {
set age_on_next_birthday [expr $this_year - $birthyear]
append html " (turns $age_on_next_birthday)"
} else {
set age_on_next_birthday [expr $this_year + 1 - $birthyear]
append html " (turns $age_on_next_birthday)"
}
}
</pre>
