Is it possible to set the value of a tag to that of another tag?

[bilatd]

May I please ask for some help in determing the command which will set the value of the "CreateDate" tag equal to the current value of the "FileCreateDate" for all files in a certain folder?

I tried the following command which did not work
exiftool "-CreateDate+=-FileCreateDate" "C:\Pictures"

I had used the tool called "Bulk File Changer" to individually set each picture in the folder with it's unique timestamp, making the equivalent of FileCreateDate and FileModifyDate equal to each other which was great. However, it appears Google Photo's reads CreateDate instead of FileCreateDate or FileModifyDate.

[StarGeek]

Use exiftool "-CreateDate<FileCreateDate" C:\Pictures

The Greater/Lesser Than symbols </> are used to copy tags.  The equal sign = is used to assign a value to a tag.

And since you mention it, Photo.Google.com will use the following tags for the date, in this order of priority:
IPTC:DateCreated+IPTC:TimeCreated
EXIF:DateTimeOriginal
XMP-photoshop:DateCreated
XMP-exif:DateTimeOriginal
IPTC:DigitalCreationDate+IPTC:DigitalCreationTime
EXIF:CreateDate
XMP-xmp:CreateDate
XMP-exif:DateTimeDigitized
EXIF:ModifyDate
XMP-xmp:ModifyDate
GPS:GPSDateStamp+GPS:GPSTimeStamp (adjusted for local timezone)
System:FileModifyDate

So I would suggest adding DateTimeOriginal, DateCreated, and TimeCreated to your command, since Google gives them a higher priority.
exiftool "-CreateDate<FileCreateDate" "-DateTimeOriginal<FileCreateDate" "-DateCreated<FileCreateDate" "-TimeCreated<FileCreateDate" C:\Pictures

[bilatd]

Thank you very much