epochdate

epochdate is a small library for storing contemporary dates (without time-of-day information) in 2 bytes per date, instead of the 16-20 bytes that are used by a time.Time value from the Go standard library.

epochdate.Date is a uint16 value representing the number of days since Jan 1 1970. The maximum representable date is Jun 6 2149. Arithmetical operations react predictably.

import (
	"github.com/extemporalgenome/epochdate"
	"time"
)

var (
	today     = epochdate.Today()
	yesterday = today - 1
	tomorrow  = today + 1
	fortnight = today + 7*2
)

Converting to Date values

assert := func(condition bool) { if !condition { panic("assertion failed") } }

d1, _ := epochdate.Parse(epochdate.RFC3339, "2012-03-10")
d2, _ := epochdate.NewFromDate(2012, 3, 10)
assert(d1 == d2)

times := []string{
	// Any time of the same day forms an equivalent epochdate value
	"2012-03-10T00:00:00Z",
	"2012-03-10T23:59:59Z",

	// And this is relative to location -- even locations that are
	// 26 hours apart (politics) will result in the same epochdate value
	// (in each location, the date at the given instant *is* 2012-03-10)
	"2012-03-10T00:00:00-12:00",
	"2012-03-10T00:00:00+14:00",
}

for _, str := range times {
	t, _ := time.Parse(time.RFC3339, str)
	d, _ := epochdate.NewFromTime(t)
	assert(d == d1)
}

t1, _ := time.Parse(time.RFC3339, "2012-03-10T00:00:00-12:00")
t2, _ := time.Parse(time.RFC3339, "2012-03-10T00:00:00+14:00")

d1, _ := epochdate.NewFromUnix(t1.Unix())
d2, _ := epochdate.NewFromUnix(t2.Unix())

// t1.Unix() returns seconds since the Unix epoch relative to UTC!
// Because t1 and t2 represent very different time instants
// (26 hours apart), don't expect passing those timestamps to NewFromUnix
// to result in the same date; unless you know what you're doing, use the
// other "New" functions instead, which normalize timezones.
assert(d1 != d2)

Converting from Date values

var (
	d, _                = epochdate.Parse(epochdate.RFC3339, "2012-03-10")
	t, _                = time.Parse(time.RFC3339, "2012-03-10T00:00:00Z")
	year1, month1, day1 = d.Date()
	year2, month2, day2 = t.Date()
)
assert(year1 == year2 && month1 == month2 && day1 == day2)
assert(t.Equal(d.UTC()) == true)

// epochdate's Local and In methods return times adjusted to midnight on
// that date in the given timezone.
assert(t.Equal(d.Local()) == false)

// So if you want the local time at the instant that it was midnight on
// that date relative to UTC, do d.UTC().Local(). Conversely, the UTC time
// at the instant that it was midnight on the given date in the local
// timezone is expressed as: d.Local().UTC()

// Date values print nicely
assert(d.String() == "2012-03-10")