Adding days to any date and even February and leap year too if any.

Here is the date i have : 2020-07-08T07:03:08Z

I need to add days to the above date with some if else conditions.

```
if(month):
    # i need to increment 30/31 days as per the month have 
    next_upcoming_date = 2020-08-07T07:03:08Z
if(year):
    # i need to increment 365/366(when leap year) as per the year 
    next_upcoming_date = 2021-07-07T07:03:08Z

```

Example: if 31st
If the subscription terms are $25.99 USD a month and the subscriber signs up on Thursday, July 31. The subscriber is billed as follows:

```
Thursday, July 31 = $25.99 USD
Saturday, August 31 = $25.99 USD
Wednesday, October 1= $25.99 USD
Saturday, November 1= $25.99 USD
and so on

```

Example: if 30th
If the subscription terms are $25.99 USD a month and the subscriber signs up on Tuesday, December 30, the subscriber is billed as follows:

```
Tuesday, December 30 = $25.99 USD
Friday, January 30 = $25.99 USD
Sunday, March 1= $25.99 USD
Wednesday, April 1= $25.99USD
and so on

```

please Suggest me how can i achieve this.