Вы можете использовать модуль datetime
:
from datetime import datetime, timedelta
x = "2018-09-23 14:46:55"
offset = "0530"
res = datetime.strptime(x, '%Y-%m-%d %H:%M:%S') + \
timedelta(hours=int(offset[:2]), minutes=int(offset[2:]))
print(res)
datetime.datetime(2018, 9, 23, 20, 16, 55)