Shane,
The start/end time in an HTT command must be in the format CCMMYYDD HHmmSS. So you can't search more precisely than a second.
The results of HTT searches will include fractions of a second that start with the end time. In other words:
HTT,AAPL,20200611 100000, 20200611 101000
Will include times that start with 101000:
HTT,AAPL,100000 20200611,100000 20200611 101000
2020-06-11
10:01:00.992868,349.4945,230,7475759,349.4600,349.5100,20424,C,19,01,0,11,
2020-06-11
10:01:00.978873,349.4850,25,7475529,349.4600,349.5100,10396,O,24,87,0,11,
2020-06-11
10:01:00.903720,349.4998,2,7475504,349.4600,349.5100,10395,O,24,87,0,11,
...etc...
2020-06-11 10:01:00.227203,349.4600,100,7473519,349.4600,349.5000,20419,C,19,01,0,11,
2020-06-11 10:01:00.221115,349.4800,100,7473419,349.4600,349.5000,8256,C,18,01,0,11,
2020-06-11 10:00:59.918933,349.4850,100,7473319,349.4600,349.5100,10390,C,24,01,0,11,
2020-06-11 10:00:59.756595,349.5100,600,7473219,349.4700,349.5100,207,C,164,01,0,11,
..etc...
2020-06-11 10:00:00.078536,349.0700,100,7353962,349.0700,349.1000,26296,C,5,01,0,11,
2020-06-11 10:00:00.006590,348.8383,213,7353862,349.0300,349.0700,20027,O,19,3E05,0,11,
!ENDMSG!,
In other words, an HTT request ending at 20200611 101200 technically ends at 20200611 102000.999999.
One other item to note: timestamp precision varies by exchange. Depending on the symbols you're requesting, they may not all go to milliseconds, in which case the same general principles apply.
Sincerely,
Gary Stephen
DTN IQFeed Implementation Support Specialist