Why do closing values on ETFs change overnight?
I track these ETFS (among many).
XLY
XLP
XLE
XLF
XLV
XLI
IYZ
XLB
XLK
XLU
XLRE
SPY
SPYG
SPYV
These are the price differences on the Wednesday close listed at roughly 1:45 a.m. ET today and 1:05 p.m. ET
Consumer Discretionary XLY -$0.01
Consumer Staples XLP $0.00
Energy XLE -$0.02
Financials XLF $0.00
Health Care Select XLV -$0.03
Industrials XLI $0.00
iShares Dow Jones Telecom IYZ $0.00
Materials XLB -$0.03
Technology XLK $0.00
Utilities XLU $0.00
Real Estate Select Sector XLRE $0.00
SPDR S&P 500 SPY -$0.05
SPDR S&P 500 Growth ETF SPYG $0.00
SPDR S&P 500 VALUE ETF SPYV $0.00
That's five of 14 in this sequence, or 35.7%.
This happens on an irregular basis but more often than I'd like. It makes the crediblity of your data (and that of your data provider) suspect.
So you can check, here is the data as of 1:45 a.m. ET
Consumer Discretionary $105.70
Consumer Staples $53.91
Energy $67.52
Financials $28.94
Health Care Select $85.80
Industrials $76.26
iShares Dow Jones Telecom $28.22
Materials $60.14
Technology $69.86
Utilities $50.23
Real Estate Select Sector $31.28
SPDR S&P 500 $275.25
SPDR S&P 500 Growth ETF $35.31
SPDR S&P 500 VALUE ETF $30.38
And the data as of 1 p.m. ET.
Consumer Discretionary $105.71
Consumer Staples $53.91
Energy $67.54
Financials $28.94
Health Care Select $85.83
Industrials $76.26
iShares Dow Jones Telecom $28.22
Materials $60.17
Technology $69.86
Utilities $50.23
Real Estate Select Sector $31.28
SPDR S&P 500 $275.30
SPDR S&P 500 Growth ETF $35.31
SPDR S&P 500 VALUE ETF $30.38
These irregularities occur often enough that I save all the daily data I collect because I am never too sure what to expect the next day. Some I can figure out, like dividends. Some divergences baffle me. And if it's because the ETF folks and/or your data provider (I assume Morningstar) or Nasdaq can't get the data collected accurately with a 4 p.m. time stamp, then you should disclose that 4 p.m. is a mere suggestion.

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Charles Blaine commented
I had hoped your disclosure page (https://help.yahoo.com/kb/finance-for-desktop/SLN2310.html?impressions=true) would address this issue, but it does not. It does not even hint at it. I would be very happy if there was an acknowledgement of this somewhere. Some things I track may never be perfect, like currencies, which trade 24/7. But the ETF issue is bothersome.