from datetime import datetime
from datetime import timedelta



t1 = datetime.now()
tstr = t1.strftime('%Y-%m-%d %H:%M:%S')
print tstr

oneday = timedelta(1)  # (day,sec,usec,msec,min,hour,week)
t = t1 + oneday
tstr = t.strftime('%Y-%m-%d %H:%M:%S')
print tstr

t = t1 - oneday
tstr = t.strftime('%Y-%m-%d %H:%M:%S')
print tstr

t1 = datetime(2018,1,1,0,0,0)
tstr = t1.strftime('%Y-%m-%d %H:%M:%S')
print tstr

t1 = t1 - oneday
tstr = t1.strftime('%Y-%m-%d %H:%M:%S')
print tstr

t2 = datetime(2018,1,2,15,2,3)
tstr = t2.strftime('%Y-%m-%d %H:%M:%S')
print tstr

t = t2 - t1
print "%d sec and %d days" %(t.seconds, t.days)
deltat = t.days*60*60*24 + t.seconds
print "total diff = %d sec" %deltat


t = t1 - t2
zero = t1 - t1
if t<zero:
  print "%d sec and %d days" %(t.seconds, t.days)
  deltat = t.days*60*60*24 + t.seconds
  print "total diff = %d sec" %deltat




==============================================

[kondo@lxplus000]~/python% python date.py
2018-05-01 09:08:06
2018-05-02 09:08:06
2018-04-30 09:08:06
2018-01-01 00:00:00
2017-12-31 00:00:00
2018-01-02 15:02:03
54123 sec and 2 days
total diff = 226923 sec
32277 sec and -3 days
total diff = -226923 sec