Python date date-str conversion and calculation of time and date

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Convert date format to string

# 根据字符串类型转日期 返回值类型<class 'time.struct_time'>
st_time = time.strptime('2019-4-30 11:32:23', '%Y-%m-%d %H:%M:%S')
print(str(st_time) + " 类型：" + str(type(st_time)))
# time.struct_time(tm_year=2019, tm_mon=4, tm_mday=30, tm_hour=11, tm_min=32, tm_sec=23, tm_wday=1, tm_yday=120, tm_isdst=-1) 类型：<class 'time.struct_time'>

# 根据<class 'time.struct_time'>日期类型转字符 返回值类型<class 'str'>
str_time = time.strftime('%Y-%m-%d %H:%M:%S',time.localtime())
print(str_time + " 类型：" + str(type(str_time)))
# 2019-04-26 12:33:04 类型：<class 'str'>

# 获取当前日期 返回值类型<class 'time.struct_time'>
st_time = time.localtime()
print(str(st_time) + " 类型：" + str(type(st_time)))
# time.struct_time(tm_year=2019, tm_mon=4, tm_mday=26, tm_hour=12, tm_min=47, tm_sec=41, tm_wday=4, tm_yday=116, tm_isdst=0) 类型：<class 'time.struct_time'>

# 获取当前日期时间戳 返回值类型<class 'float'>
float_time = time.time()
print(str(float_time) + " 类型：" + str(type(float_time)))
# 1556254126.6432147 类型：<class 'float'>

# 根据<class 'time.struct_time'>日期类型转换为时间戳 返回值类型<class 'float'>
float_time = time.mktime(time.localtime())
print(str(float_time) + " 类型：" + str(type(float_time)))
# 1556256409.0 类型：<class 'float'>

# 根据<class 'float'>日期戳转换为struct_time日期 返回值类型<class 'time.struct_time'>
st_time = time.localtime(time.time())
print(str(st_time) + " 类型：" + str(type(st_time)))
# time.struct_time(tm_year=2019, tm_mon=4, tm_mday=26, tm_hour=15, tm_min=5, tm_sec=48, tm_wday=4, tm_yday=116, tm_isdst=0) 类型：<class 'time.struct_time'>

Calculation of time and date

Highly recommend relativedelta

The relativedelta type is intended to be applied to an existing date-time, and can replace specific components of that date-time, or represent time intervals.
Simple to use:

There are two different ways to construct a relativedelta instance. The first is to pass two date/datetime classes:

eg:relativedelta(datetime1, datetime2)

The second is to pass any number of the following keyword arguments:

relativedelta(arg1=x,arg2=y,arg3=z...)

year, month, day, hour, minute, second, microsecond:
    Absolute information (argument is singular); adding or subtracting a
    relativedelta with absolute information does not perform an arithmetic
    operation, but rather REPLACES the corresponding value in the
    original datetime with the value(s) in relativedelta.

years, months, weeks, days, hours, minutes, seconds, microseconds:
    Relative information, may be negative (argument is plural); adding
    or subtracting a relativedelta with relative information performs
    the corresponding arithmetic operation on the original datetime value
    with the information in the relativedelta.

weekday: 
    One of the weekday instances (MO, TU, etc) available in the
    relativedelta module. These instances may receive a parameter N,
    specifying the Nth weekday, which could be positive or negative
    (like MO(+1) or MO(-2)). Not specifying it is the same as specifying
    +1. You can also use an integer, where 0=MO. This argument is always
    relative e.g. if the calculated date is already Monday, using MO(1)
    or MO(-1) won't change the day. To effectively make it absolute, use
    it in combination with the day argument (e.g. day=1, MO(1) for first
    Monday of the month).

leapdays:
    Will add given days to the date found, if year is a leap
    year, and the date found is post 28 of february.

yearday, nlyearday:
    Set the yearday or the non-leap year day (jump leap days).
    These are converted to day/month/leapdays information.

Keyword arguments have relative and absolute forms. The plural is relative and the singular is absolute. For each parameter in the following order, the absolute form is applied first (by setting each property to that value), and then the relative form is applied (by adding the value to the property).
The order of properties considered when adding this relativedelta to a datetime is:
Year
Month
Day
Hour
Minute
Second
Microsecond
Finally, the weekday is applied using the above rules.
For example:

>>> from datetime import datetime
>>> from dateutil.relativedelta import relativedelta, MO
>>> dt = datetime(2018, 4, 9, 13, 37, 0)
>>> delta = relativedelta(hours=25, day=1, weekday=MO(1))
>>> dt + delta
datetime.datetime(2018, 4, 2, 14, 37)

使用示例：
date_end = date_start.date() + relativedelta(months=3, days=-1)

Return a dateutil.relativedelta.relativedelta object.

timedelta is an object in datetime that represents the difference between two times

Constructor: datetime.timedelta(days=0, seconds=0, microseconds=0, milliseconds=0, minutes=0, hours=0, weeks=0) where the parameters are optional and the default value is 0
where
:

1 millisecond = 1000 microseconds
1 minute = 60 seconds
1 hour = 3600 seconds
1 week = 7 days

In the constructor, pay attention to the range of parameter values:

0 <= microseconds < 1000000
0 <= seconds < 3600*24 (the number of seconds in one day)
-999999999 <= days <= 999999999

import datetime
 
nowtime=datetime.datetime.now()
 
print(nowtime.strftime('%Y-%m-%d %H:%M:%S'))
 
delta=datetime.timedelta(days=1)
 
rs=nowtime+delta
 
print(rs.strftime('%Y-%m-%d %H:%M:%S'))