I'm getting timestamps off by about 1 hour.
- x&& = ExtendedTimer 
- y&& = epoch 
-   
-     l1  = INSTR(- l  + 1,-  d$ , "-")
-         SELECT CASE-  i  'Add the number of days for each previous month passed
 
-             CASE 1- : d  =-  d  'January doestn't have any carry over days.
 
-             CASE 2, 4, 6, 8, 9, 11- : d  =-  d  + 31
 
-             CASE 5, 7, 10, 12- : d  =-  d  + 30
 
-         d = d + 365 
-         IF-  m  > 2 THEN-  d  =-  d  + 1 'add an extra day for leap year every 4 years, starting in 1970
 
-     d = d - 1 'for year 2000 
-     s~&& = d * 24 * 60 * 60 'Seconds are days * 24 hours * 60 minutes * 60 seconds 
-     ExtendedTimer##  = (- s~&&  + TIMER)
-   
-   
-   
-   
-     '--get days passed since 1-1-1970-- 
-     total_past_days = year_day_count + leap_year_days 
-     total_past_sec = total_past_days * 86400 
-   
-     '--get days so far in month-------- 
-     mc(1) = 31 
-     mc(2) = 28 
-     mc(3) = 31 
-     mc(4) = 30 
-     mc(5) = 31 
-     mc(6) = 30 
-     mc(7) = 31 
-     mc(8) = 31 
-     mc(9) = 30 
-     mc(10) = 31 
-     mc(11) = 30 
-     mc(12) = 31 
-     '--get days past to month start---- 
-         x = x + 1 
-         day_count = day_count + mc(x) 
-     current_month_sec = day_count * 86400 
-   
-     month_time_sec = month_days * 86400 
-   
-     '--get current seconds in day------ 
-     day_time_sec = hours + mins + sec 
-   
-     epoch = total_past_sec + month_time_sec + current_month_sec + day_time_sec 
-   
(ExtendedTimer can be found in the sample area here on the forums: 
https://www.qb64.org/forum/index.php?topic=1598.0 )
Checking the timestamps against the website here, your values appear to be off by about an hour: 
https://www.epochconverter.com/Looking at the code, is this intentional?       hours = (VAL(LEFT$(TIME$, 2)) - 1) * 3600 
It seems as if you're subtracting an hour from the timer intentionally?  If this is for a timezone localization, you might want to make a note of that in your post and make it a variable which another user could set to adjust for their own timezone as needed.