Read this if you are interested in selecting photons arrived at a particular time period (e.g. in analysis of flares, or for phase resolved spectroscopy).
You should define Good Time Intervals (GTIs) with the help of the gti_user program. To create a GTI for IBIS starting on IJD 1322.68 and lasting 1 minute (do not forget to convert it into days!) give the command:
gti_user gti=user_gti.fits begin=1322.68 end= \
length=6.944E-4 group=og_ibis.fits\ unit='day'
Note that the parameter unit governs simultaneously begin, end, length and step parameters. If unit='day', then begin and end are considered to be absolute values in IJD. If unit='sec' then, begin and end are taken relative to "tstart" (See the full list of gti_user parameters in [1]).
It is also possible to define the GTIs of the fixed length (parameter length), separated by the given step (parameter step). In the example below 6 GTIs of 0.1 day length, beginning at time IJD 1092.5. and separated by 0.2 days are defined.
gti_user gti=user_gti_repeat.fits begin=1092.5 end= \ length=0.1 repeat=5 step=0.2 group=mygroup.fits\
Then in the main page of the GUI, Fig. 12, you should set SCW1_GTI_gtiUserI="user_gti.fits", and specify the time format that was used in this file, in the example above SCW1_GTI_TimeFormat="IJD". More details on gti_user are in the Data Analysis section of [1].