This command is used to compute the floating point time interval in seconds from a start time in localtime 11 format. The optional end value defaults to the current time.
The command is part of the Humelib package and can be imported from the namespace humelib.
package require Humelib
namespace import -force humelib::*
set start [localtime 11]
tk_dialog .stopwatch {Timer Running} {Press Ok when done.} info 0 Ok
set seconds [timediff $start]
puts "elapsed time=$seconds"