If these dates are entered in your spreadsheet as text, you will need to use text replacement functions. Luckily, Google Spreadsheets supports find & replace with regular expressions. So:
- Click Edit → Find and replace.
- Check the Search using regular expressions checkbox
- In the Find field, enter
- In the Replace with field, enter
Click Replace (or Replace all, if you're feeling lucky).
The expression entered into the Find field is a regular expression, which matches 1) a number:
\d+, 2) a dash
-, 3) another number
\d+, and so on. It is a pattern for a datetime in your format:
YYYY-MM-DD HH:MM:SS +ZZZZ. Note that the regular expression has parenthesizes around the date/time part, but not around the timezone part.
The expression in the Replace with field says "take the first group (enclosed in parenthesizes),
$1, and append
-0700 to it".
If you want to learn more about using regular expressions (which can be useful in many situations), there are many good resources on line. These are some good ones: