虽然我希望有一个简短的gnuplot 集成"解决方案，但我通过定义自己的公式提出了一个有点冗长"的解决方案.

That's a somewhat "lengthy" solution I have come up with by defining my own formula, although I was hoping for a short "gnuplot-integrated" solution.

代码:

### change 12h time format to 24h format
reset session

$Data12 <<EOD
12/31/19  8:12:01 AM
12/31/19  8:12:02 PM
12/31/19 12:00:03 am
12/31/19 12:00:04 pm
EOD

myTimeFmt12a = "%m/%d/%y"
myTimeFmt12b = "%H:%M:%S"
myTimeFmt24 = "%d.%m.%Y %H:%M:%S"

# change 12h am/pm format to 24h format
myTime12to24(t,p) = t+12*3600*(floor(t/3600)<12 && (p eq "PM" || p eq "pm") ? 
                    1 : floor(t/3600)==12 && (p eq "AM" || p eq "am")  ? -1 : 0)

set table $Data24
    plot $Data12 u (strftime(myTimeFmt24,timecolumn(1,myTimeFmt12a) + 
         myTime12to24(timecolumn(2,myTimeFmt12b),strcol(3)))) w table
unset table
print $Data24
### end of code

结果:

 31.12.2019 08:12:01    
 31.12.2019 20:12:02    
 31.12.2019 00:00:03    
 31.12.2019 12:00:04