Use =A1*86400
if the time is in A1.
Time values are treated as the numerical ratio of the time under discusson and one full day (24*60*60 s)
by spreadsheets. With different words: The implicit unit used is 1 d.
Thus 1d = 24 h = 24* 60 min = 24 * 60 * 60 s = 86400 s
Your examples (slightly enhanced the second one):
2:02:48 = 0.0852777777777778 numerically in Calc meaning 2 h 2 min 48 s = 7368 s
00:00:37.534 = 0.000434421296296296 numerically meaning 37.534 s
Please note that time values handled under the standards of Calc (in days) cannot be added or otherwise put together with the non-standard time values expressed in seconds without a conversion by *86400
or /86400
respectively.
Edit (regarding the remarks by the OQ in his self-answer):
The numerical result for =(A1-A2)*86400 you get is surely the correct number of seconds for the duration. It is just badly formatted (probably by one of the many silly smart automatisms, probably by yourself unintentionally). I got the result shown correctly using the same formula (in a virgin cell).
Being an integer this result formatted to time (TOD) format will show 00:00:00
as any other integer also would. Remember my explanations about the handling of date-time in spreadsheets. The time part of date-time values always is coded in the fractional part of the numerical value which obviously is 0 for the correct result of 1642
as it is for any integer. If Calc badly formatted the cell, just reformat it to your preferred variant from the ‘Number’ list, and you will get shown 1642
or something equivalent.
By the way: The date-time values you posted here are in an unusual format DD-MM-YYYY HH:MM:SS
I would advise against. I know that there are many regional preferrences (mostly very doubtable), but the dash-delimited date format you use might not be due to “locale affairs”. Why not use the well designed ISO 8601 format YYYY-MM-DD
?