It only applies to the amounts earned during marriage, and income on assets comingled after the marriage.  If you live in a community property state, you will generally split it all 50/50. The exception would be income earned on assets or investments that were owned by a spouse prior to the marriage and not commingled. Also, inherited income that was not commingled. 

Also, when you file as Married Filing Separately, if one spouse itemizes on Schedule A, the other must also itemize on Schedule A.

Married Filing Separately in a Community Property State

https://ttlc.intuit.com/replies/3301943

Is it better for a married couple to file jointly or separately?

https://ttlc.intuit.com/replies/3288477

If it is only "during a marriage," then does that mean we must calculate the prorated wages and federal tax withholding amounts as measured from the date of the marriage? For example, if married in September 1, you would need to take your wages and multiply that by .25 (Sept-Dec = 4 months which is .25 of a year), then repeat the same process for tax withholding? If so, it would seem this also gets more complicated if you got married on a date that does not cleanly fall on a rounded number, like September 19 or something, correct?