How do I apply ensure that the "lastModificationTime" date is always today in FKM?

You can create a rule comparing the last time stamp of the modification with the current time of day.

set $(timestamp) parseDate("%a %b %d %H:%M:%S %Y", value)

if $(timestamp) < startOfDay() then

severity warning

else

severity ok

endif

This modification checks if the last time stamp is recorded before midnight; if not, then warns user if it already has passed midnight.