Deltime computes the elapsed time between now and a specified date/time, or between two specified date/times. The format for specifying date/times is pretty loose - basically the same as the format for date/times in network mail. Just be careful to put them in quotes if they contain spaces. The output format is dddd hh:mm:ss.
Times earlier than 1970 can be handled, because the internal Unix* time format is not used. However, time spans greater than 66 years cannot be handled, because that's 2**31 seconds.