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### 40.10 Time Calculations

These functions perform calendrical computations using time values (see Time of Day). As with any time value, a value of `nil` for any of their time-value arguments stands for the current system time, and a single number stands for the number of seconds since the epoch.

Function: time-less-p t1 t2

This returns `t` if time value t1 is less than time value t2. The result is `nil` if either argument is a NaN.

Function: time-equal-p t1 t2

This returns `t` if t1 and t2 are equal time values. The result is `nil` if either argument is a NaN.

Function: time-subtract t1 t2

This returns the time difference t1 - t2 between two time values, as a Lisp time value. The result is exact and its clock resolution is no worse than the worse of its two arguments’ resolutions. The result is floating-point only if it is infinite or a NaN. If you need the difference in units of elapsed seconds, you can convert it with `time-convert` or `float-time`. See Time Conversion.

This returns the sum of two time values, using the same conversion rules as `time-subtract`. One argument should represent a time difference rather than a point in time, as a time value that is often just a single number of elapsed seconds. Here is how to add a number of seconds to a time value:

```(time-add time seconds)
```
Function: time-to-days time-value

This function returns the number of days between the beginning of year 1 and time-value, assuming the default time zone. The operating system limits the range of time and zone values.

Function: time-to-day-in-year time-value

This returns the day number within the year corresponding to time-value, assuming the default time zone. The operating system limits the range of time and zone values.

Function: date-leap-year-p year

This function returns `t` if year is a leap year.

Function: date-days-in-month year month

Return the number of days in month in year. For instance, February 2020 has 29 days.

Function: date-ordinal-to-time year ordinal

Return the date of ordinal in year as a decoded time structure. For instance, the 120th day in 2004 is April 29th.

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