Conditional statements with time as parameter


There are missing values for the depth_m column that I would like to fill in based on time. The conditional statement I’ve written is:
if_else(index_recorded_timestamp <= 1997-08-08 19:00:00 & index_recorded_timestamp => 1997-08-08 18:50:00, depth_m = 400), depth_m

The error message I get is:
Left parenthesis ‘(’ without matching right parenthesis ‘)’ is found. Maybe ‘)’ at the end is missing?

But I think it’s not that simple. I’ve tried using ifelse and I’ve tried specifying date as.posixct. Ultimately I will write a set of nested if/then statements, but think I can do that if I can get a little help starting out. Thanks in advance!