How to convert Datetime to Unix timestamp in OCaml

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    open Unix

    (* Convert Date to Unix timestamp *)
    let tm = {
        tm_sec = 45;
        tm_min = 30;
        tm_hour = 8;
        tm_mday = 15;
        tm_mon = 4;   (* May is the 5th month, but zero-based, so 4 *)
        tm_year = 120; (* 2020 - 1900 *)
        tm_wday = 0;  (* Ignored by mktime *)
        tm_yday = 0;  (* Ignored by mktime *)
        tm_isdst = false;  (* Not using daylight saving time *)
    }
    
    let (unix_timestamp, _) = Unix.mktime tm
    
    let () =
        Printf.printf "Unix Timestamp: %.0f\n" unix_timestamp

Output:

Unix Timestamp: 1586939445

Example only. There may be multiple ways to perform this operation.

Code Explanation

In this Ocaml example, a tm record is created for the date '2020-04-15 08:30:45'. The Unix.mktime function converts this tm record into a Unix timestamp. The output will be the Unix timestamp '1586939445', which is printed using Printf.printf.
The tm_mon and tm_year fields are adjusted (months are zero-based, and the year is counted from 1900).

Convert Unix timestamp to Date in Ocaml

Other useful OCaml date functions

Function	Description	Example
`Unix.time()`	Returns the current Unix timestamp (seconds since 1970-01-01).	`Unix.time()` returns 1693227296.0
`Unix.gmtime()`	Converts a Unix timestamp to a `tm` structure in UTC.	`Unix.gmtime(1589530245.0)` returns the corresponding date and time in UTC.
`Unix.localtime()`	Converts a Unix timestamp to a `tm` structure in the local time zone.	`Unix.localtime(1589530245.0)` returns the date and time in the local time zone.
`Unix.mktime()`	Converts a `tm` structure to a Unix timestamp.	`Unix.mktime(tm)` converts a `tm` structure to a Unix timestamp.
`Unix.strftime()`	Formats a `tm` structure as a string according to the specified format.	`Unix.strftime "%Y-%m-%d" tm` returns "2020-05-15"
`Unix.strptime()`	Parses a date string into a `tm` structure using a format string.	`Unix.strptime "%Y-%m-%d" "2020-05-15"` parses the string into a `tm` structure.
`Unix.sleep()`	Suspends the execution of the program for the specified number of seconds.	`Unix.sleep(5)` suspends the program for 5 seconds.
`Unix.gettimeofday()`	Returns the current time as a floating-point number of seconds since the epoch, including fractional seconds.	`Unix.gettimeofday()` returns 1693227296.123

Current Unix Timestamp SECONDS SINCE JAN 01 1970

Current UTC Time

2025-08-20