How to calculate epoch day?

Question

Is calculating the epoch day as simple as taking the epoch seconds and dividing by 86400? Or are there some special calculations that need to be done to take account of daylight savings or leap year or some other factor?

Update: by "epoch day" I mean number of days since the epoch.

Answer 1

POSIX defines that you can deduce the number of days since The Epoch (1970-01-01 00:00:00Z) by dividing the timestamp by 86400. This deliberately and consciously ignores leap seconds.

See the definition Seconds since the Epoch:

4.15 Seconds Since the Epoch

A value that approximates the number of seconds that have elapsed since the Epoch. A Coordinated Universal Time name (specified in terms of seconds (tm_sec), minutes (tm_min), hours (tm_hour), days since January 1 of the year (tm_yday), and calendar year minus 1900 (tm_year)) is related to a time represented as seconds since the Epoch, according to the expression below.

If the year is <1970 or the value is negative, the relationship is undefined. If the year is >=1970 and the value is non-negative, the value is related to a Coordinated Universal Time name according to the C-language expression, where tm_sec, tm_min, tm_hour, tm_yday, and tm_year are all integer types:

tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +
     (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 -
     ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400

The relationship between the actual time of day and the current value for seconds since the Epoch is unspecified.

How any changes to the value of seconds since the Epoch are made to align to a desired relationship with the current actual time is implementation-defined. As represented in seconds since the Epoch, each and every day shall be accounted for by exactly 86400 seconds.

Note: The last three terms of the expression add in a day for each year that follows a leap year starting with the first leap year since the Epoch. The first term adds a day every 4 years starting in 1973, the second subtracts a day back out every 100 years starting in 2001, and the third adds a day back in every 400 years starting in 2001. The divisions in the formula are integer divisions; that is, the remainder is discarded leaving only the integer quotient.