solution and changes for issue #3519

[matejtuke]

Issue fix #3519. All tests were successfully passed.

For this issue, I made the following changes:

1. subDays/test.ts

• - Added a test check for subtracting through UTC time.

2. addDays/test.ts

• - Added a test check for adding through UTC time.
• - Deleted offset calculation because it is no longer necessary for the new calculation in addDays/index.ts. (This calculation was used with 4 tests)
• - Deleted the MINUTE variable because it is not used now.

3. addDays/index.ts

• - Changed the calculation algorithm from _date.setDate(_date.getDate() + amount) to a new calculation based on milliseconds. (As was described in the issue description)

4. addWeeks/index.ts

• - Deleted the old calculation based on addDays() function because this function has new calculation logic now. Changed its calculation for its old algorithm logic _date.setDate(_date.getDate() + amount * 7). In fact, there was only 1 line added.
• - Deleted the import for addDays(), because it is not used now.

5. nextMondays/tests.ts

• - Only minor editing of details to make the code consistent. Changed months in the dates so they would be consistent with other functions such as: nextTuesday(), nextWednesday(), nextThursday(), nextFriday(), nextSaturday(), nextSunday().