Dear Oluwaseun,
If your time axis is annual climatology (that is, if it repeats itself every year), it should be defined as "modulo". Then, you can avoid these white bands by extending the data beyond the beginning and end of the year, as in
fill/hlimits=0:365/ . . . pr[t=-40:400, . . .]
if the underlying units of time is "days" and if your climatological year consists of 365 days. Basically, you limit the region of plotting to one year, but ask Ferret to take values before and after the year into account when generating contour lines (shadings). Having said that, I'm not too familiar with this issue and I would always need some trial and error before I get the right results.
To see what time axis your data has, you want to see the outputs of these commands:
yes? show grid pr
yes? list t[gt=pr]
Hope this helps.
Ryo