diff --git a/doc/Doc.md b/doc/Doc.md
index 6391235..9cc3d69 100644
--- a/doc/Doc.md
+++ b/doc/Doc.md
@@ -70,6 +70,16 @@
  => "2021-03-13 19:53:01"`|
 |`(str-to-date string format)`|Converst string to time of secs since epoch. |`>>> (str-to-date "2021-03-13 19:53:01" "%Y-%m-%d %H:%M:%S") => 1615665181`|
 |`(date-add date amount unit)`|Add number of units to date. A unit is one of 'year', 'month', 'day', 'hour', 'minute' or 'second'|`>>> (date-to-str (date-add (str-to-date "2021-03-13 19:53:01" "%Y-%m-%d %H:%M:%S") 10 "day") "%Y-%m-%d %H:%M:%S") => "2021-03-23 20:53:01"`|
+|`start-of-day datetime`||`>>> (start-of-day (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S")) => 1620864000`|
+|`end-of-day datetime`||`>>> (end-of-day (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S")) => 1620950399`|
+|`start-of-month datetime`||`>>> (start-of-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S")) => 1619827200`|
+|`start-of-next-month datetime`||`>>> (end-of-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S")) => 1622505599`|
+|`end-of-next-month datetime`||`>>> (start-of-next-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S")) => 1622505600`|
+|`end-of-month datetime`||`>>> (end-of-next-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S")) => 1625097599`|
+|`start-of-prev-month datetime`||`>>> (start-of-prev-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S")) => 1617235200`|
+|`end-of-prev-month datetime`||`>>> (end-of-prev-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S")) => 1619827199`|
+|`start-of-year datetime`||`>>> (start-of-year (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S")) => 1609459200`|
+|`end-of-year datetime`||`>>> (end-of-year (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S")) => 1640995199`|
 |`(debug ..)`|||
 |`(display ..)`|||
 |`(string-replace source substr replacement)`|Replace a substring with a replacement string in a source string|`>>> (string-replace "abcdefg" "de" "DE") => "abcDEfg"`|
diff --git a/stdlib/stdlib.lsp b/stdlib/stdlib.lsp
index 3b0b775..3447dfb 100644
--- a/stdlib/stdlib.lsp
+++ b/stdlib/stdlib.lsp
@@ -135,6 +135,16 @@
     (quick-sort-by l (lambda (a b) (> a b))))
 
 
+(defun start-of-day (datetime) (str-to-date (+ (date-to-str datetime "%Y-%m-%d") " " "00:00:00") "%Y-%m-%d %H:%M:%S"))
+(defun end-of-day (datetime) (str-to-date (+ (date-to-str datetime "%Y-%m-%d") " " "23:59:59") "%Y-%m-%d %H:%M:%S"))
+(defun start-of-month (datetime) (str-to-date (+ (date-to-str datetime "%Y-%m") "-01 00:00:00") "%Y-%m-%d %H:%M:%S"))
+(defun start-of-next-month (datetime) (date-add (start-of-month datetime) 1 "month"))
+(defun end-of-next-month (datetime) (date-add (end-of-month datetime) 1 "month"))
+(defun end-of-month (datetime) (date-add (date-add (start-of-month datetime) 1 "month") -1 "second"))
+(defun start-of-prev-month (datetime) (date-add (start-of-month datetime) -1 "month"))
+(defun end-of-prev-month (datetime) (date-add (end-of-month datetime) -1 "month"))
+(defun start-of-year (datetime) (str-to-date (+ (date-to-str datetime "%Y") "-01-01 00:00:00") "%Y-%m-%d %H:%M:%S"))
+(defun end-of-year (datetime) (str-to-date (+ (date-to-str datetime "%Y") "-12-31 23:59:59") "%Y-%m-%d %H:%M:%S"))
 
 
 ; from list of lists creates csv string
diff --git a/tests/test.lsp b/tests/test.lsp
index d01012c..bec07a5 100644
--- a/tests/test.lsp
+++ b/tests/test.lsp
@@ -96,6 +96,19 @@
 
 (ut::define-test "result of (!= nil nil)" '(ut::assert-false (!= nil nil)))
 
+
+(ut::define-test "result of (start-of-day)" '(ut::assert-equal 1620864000 (start-of-day (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S"))))
+(ut::define-test "result of (end-of-day)" '(ut::assert-equal 1620950399 (end-of-day (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S"))))
+(ut::define-test "result of (start-of-month)" '(ut::assert-equal 1619827200 (start-of-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S"))))
+(ut::define-test "result of (end-of-month)" '(ut::assert-equal 1622505599 (end-of-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S"))))
+(ut::define-test "result of (start-of-next-month)" '(ut::assert-equal 1622505600 (start-of-next-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S"))))
+(ut::define-test "result of (end-of-next-month)" '(ut::assert-equal 1625097599 (end-of-next-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S"))))
+(ut::define-test "result of (start-of-prev-month)" '(ut::assert-equal 1617235200 (start-of-prev-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S"))))
+(ut::define-test "result of (end-of-prev-month)" '(ut::assert-equal 1619827199 (end-of-prev-month (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S"))))
+(ut::define-test "result of (start-of-year)" '(ut::assert-equal 1609459200 (start-of-year (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S"))))
+(ut::define-test "result of (end-of-year)" '(ut::assert-equal 1640995199 (end-of-year (str-to-date "2021-05-13 10:32:12" "%Y-%m-%d %H:%M:%S"))))
+
+
 ;(ut::define-test "result of " '(ut::assert-true )
 (ut::run-tests)