Date/Time Original is the original date of a photo: the date and time the picture was taken. It is an EXIF field with the tag name DateTimeOriginal and the tag ID 0x9003, or 36867 in decimal. A camera or phone records it when you take the picture.
Date/Time Original is stored inside the file, so it stays with the photo when you copy it. The created and modified dates that a folder lists come from the file system instead, for example File Modification Date/Time.
Which files carry it
EXIF data is found in JPG, TIFF, PNG, and WebP images. Many raw camera files that are built on TIFF carry EXIF data as well. HEIC photos can carry an EXIF block too, and so can some AVI and MOV videos.
XMP, another metadata format, has a field with the same name, DateTimeOriginal. MP4 and MOV videos have QuickTime date fields, such as Create Date, and ExifTool also lists a QuickTime DateTimeOriginal tag. The QuickTime specification says its main date fields should be stored in UTC, but many cameras store local time there instead.
What the value looks like
The EXIF standard sets one fixed format for the value. It starts with the year, month, and day, separated by colons, then a space and the time in 24-hour format. The full value has 19 characters, for example:
2018:05:28 05:58:42means 28 May 2018, at 5:58:42 in the morning1997:09:01 12:00:00is the example the EXIF standard itself uses: 1 September 1997, at 12 noon
The colons in the date are part of the standard, not an error. If the camera does not know the date, the standard says to fill every position except the colons with spaces. A blank value means that the date is unknown.
Time zone and fractions of a second
The value itself carries no time zone. It shows the local time on the camera clock. Cameras set to different time zones record different values for the same moment.
Exif version 2.31, published in July 2016, added the field OffsetTimeOriginal. It stores the difference from UTC, including daylight saving time. The format is a plus or minus sign, then hours and minutes, for example -05:00.
A second field, SubSecTimeOriginal, adds fractions of a second. A value of 130 in this field means 0.130 seconds after the time in Date/Time Original.
Where the value comes from
Date/Time Original is one of 3 main date and time fields in EXIF. CreateDate, which the standard calls DateTimeDigitized, records when the image was stored as digital data. ModifyDate records when the file was last changed.
If the camera saves the file when it takes the photo, Date/Time Original and Create Date hold the same value. They can differ when the image becomes digital data later, for example when an old print is scanned.
Why it matters
Windows treats the date taken as the main date of a photo. Its Date Taken property comes from the EXIF data and uses the number 36867, the tag ID of Date/Time Original.
The value is only as correct as the camera clock. If you travel to another time zone and do not change the clock, every photo carries your home time. Programs can also write a new value into this field, so it does not prove when a photo was taken.
Before you share photos in public, check this field together with the GPS rows. The two can show when and where you took each picture.