Some Date examples

# See "long" root's sessions
last root -t $(date -d "-100000 sec" +%Y%m%d%H%M%S)|grep still

# See timezone offset
date +%z

# See future timezone offset
date -d "+31 day" +%z

# See time after 31 day
date -d "+31 day"

# In UTC
date -d "+31 day" -u

# UNIX timestamp (since 1970)
date +%s
date +%s.%N # with nanosecs

# UNIX timestamp to date
date --date=@1239646412

# Current month
mon=$(date +%m)

# Month begin
beg=$(date +%s -d "${mon}/1")

# Current time
now=$(date +%s)

# Month end
end=$(date +%s -d "${mon}/1+ 1 month - 1 sec")

# Month percent
echo "(${now}-${beg})/(${end}-${beg})*100"|bc -lq

# Date of file modification
date -r file +%s