Calculating elapsed time from existing text

    wave/T w_Start_Time = $s_Current_DF + "start_time"
    wave/T w_End_Time = $s_Current_DF + "end_time"
 
    string s_Start_Time = w_Start_Time[0]
    string s_Start_Year = s_Start_Time[0,3]
    string s_Start_Month = s_Start_Time[5,6]
    string s_Start_Day = s_Start_Time[8,9]
    string s_Start_Hour = s_Start_Time[11,12]
    string s_Start_Minute = s_Start_Time[14,15]
    string s_Start_Second = s_Start_Time[17,25]
 
    string s_End_Time = w_End_Time[0]
    string s_End_Year = s_End_Time[0,3]
    string s_End_Month = s_End_Time[5,6]
    string s_End_Day = s_End_Time[8,9]
    string s_End_Hour = s_End_Time[11,12]
    string s_End_Minute = s_End_Time[14,15]
    string s_End_Second = s_End_Time[17,25]
    
    variable v_Measurement_Time_Start = date2secs(str2num(s_Start_Year), str2num(s_Start_Month), str2num(s_Start_Day)) + str2num(s_Start_Hour)*60*60 + str2num(s_Start_Minute)*60 + str2num(s_Start_Second)
    variable v_Measurement_Time_End = date2secs(str2num(s_End_Year), str2num(s_End_Month), str2num(s_End_Day)) + str2num(s_End_Hour)*60*60 + str2num(s_End_Minute)*60 + str2num(s_End_Second)
    variable v_Measurement_Time = v_Measurement_Time_End - v_Measurement_Time_Start

function ISO8601datetime2secs(string str)
    variable year, month, day, hh, mm, ss
    sscanf str, "%d-%d-%d%*[Tt ]%d:%d:%f", year, month, day, hh, mm, ss
    return v_flag == 6 ? date2secs(year, month, day) + ss + 60*mm + 60*60*hh : NaN
end