determining the exact time of turning on?

[whakamaru]

nb,  Old DOS clone. with no CMOS clock
I've heard that it is possible to find the time of turn on from a PC left running in Word WP for a day and a half, by rebooting and then examining the temporary file created by Word at start up, which apparently has a time/date stamp. 
How is that done? 
It's not what I would do.  That  destroys the tick count,  I would exit Word, then type TIME, and DATE.  And maybe go to debug and look at 0040:006c 6d 6e 6f for the tick count.

[FORTRANS]

Hi,

   If you really do not have a clock in your computer, it cannot
have the correct time unless you (or something else) tells it
what the time is.  Looking at a file's creation (or last modified)
time tells you only when the file was created.  It says nothing
about when you booted up.  You should prompt the user for
the date and time in the AUTOEXEC.BAT or dial out to an
external time server.

Regards,

Steve N.

[Tedd]

If there's no CMOS clock, where does the time reference for the date-time stamp come from?

Anyway, to find the startup time, you need some kind of time reference (if there's no clock then file time stamps are useless) and then use that with the current tick count.

[dedndave]

BIOS uses one of the counters to keep a clock tick - DOS keeps the date in memory
the clock tick rolls over at 1800B0h - DOS updates the date

if you do not set the date (and it isn't set by a clock driver), it thinks it is Jan 1, 1980
which is absurd, of course
everyone knows the first PC wasn't made until 1981   :P