# 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.

### functiontime-less-pt1 t2β

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

### functiontime-equal-pt1 t2β

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

### functiontime-subtractt1 t2β

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 argumentsβ 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:

### functiontime-to-daystime-valueβ

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.

### functiontime-to-day-in-yeartime-valueβ

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.

### functiondate-leap-year-pyearβ

This function returns t if year is a leap year.

### functiondate-days-in-monthyear monthβ

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

### functiondate-ordinal-to-timeyear ordinalβ

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